Rockford Fosgate Prime R2-300X4 Review: Easy Setup, Modest Power

The Rockford Fosgate Prime R2-300X4 is a compact Class D amplifier for a factory-radio upgrade or a four-door-speaker system, and this Rockford Fosgate Prime R2-300X4 review finds it worth buying if easy setup and a clear gain-setting method matter more to you than rated power. Rockford Fosgate rates it at 50 watts RMS per channel at 4 ohms, 75 watts at 2 ohms and 150 watts bridged at 4 ohms. Rockford Fosgate still sells it as part of the third-generation Prime R2 range, and no successor model is confirmed.

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Rockford Fosgate Prime R2-300X4 design and build

The owner’s manual gives the amplifier a height of 1.9 in and a 6.8 in by 10.0 in footprint, while the spec sheet rounds the long side to 10.1 in. The manual also lists a 7.3 in overall length, so measure the mounting spot with that figure in mind. Rockford Fosgate describes a cast aluminum heat sink with convection cooling and asks for at least 1 in of air gap around it. Mounting in the engine compartment voids the warranty, according to the manual.

Protection is built in. Rockford Fosgate lists short-circuit and over-current protection, and its NOMAD circuit is described as guarding against shorted speaker wires and dangerously low impedance. A red Protect LED lights when the amplifier overheats or sees a short, and the output mutes. The manual says the power and ground terminals accept wire up to 4 AWG, the speaker terminals and remote terminal take up to 8 AWG, and a 3 mm Allen wrench is included for the set screws.

R2-300X4 features and usability

Inputs are the amplifier’s strongest practical point. It has front and rear RCA inputs with balanced differential circuitry, plus front and rear high-level inputs on 4-pin Molex plugs. The manual lists Molex pigtail connectors in the box, so a radio with only speaker-level outputs can feed the amplifier directly. Use either high-level or RCA inputs, never both. A 2/4 channel switch matches the amplifier to one or two input pairs, and a single input pair goes into the front inputs with the switch on 2. RCA pass-through outputs let you feed a second amplifier, and Rockford Fosgate says the pass-through works with either RCA or high-level input.

Each channel pair has its own crossover with a high-pass, low-pass or all-pass setting. The slope is 12 dB per octave (Butterworth) and the frequency runs from 50 Hz to 250 Hz. The manual suggests 50 Hz to 80 Hz for subwoofers and 80 Hz to 120 Hz for midrange drivers from 5.25 in to 6x9 in. Auto turn-on uses the DC offset on the radio’s front-left speaker lead, so it only works with high-level inputs. With RCA inputs, wire the remote lead.

Gain setting uses Rockford Fosgate’s C.L.E.A.N. indicators. The manual’s method runs in this order:

1. Disconnect the speakers from the amplifier outputs and set both gains to 1.
2. Set the radio’s tone controls flat and loudness off, then raise the radio volume until the C.L.E.A.N. IN light turns red. Back off until it goes dark.
3. At that radio volume, raise the amplifier gain until C.L.E.A.N. OUT turns red, then back off until it goes dark.
4. If you use test tones instead of music, the manual suggests 1 kHz at -5 dB for midrange drivers and 40 Hz at -10 dB for subwoofers.

Several features common on rival amplifiers are missing. The manual lists no remote level control, no infrasonic filter and no Punch EQ bass boost for this model, because those belong to the mono amplifiers in the range. Some retailer descriptions repeat a Punch EQ claim, so treat that line as an error.

Rockford Fosgate Prime R2-300X4 rated power and calculated figures

Our research found no independent lab measurements for this model. Everything below therefore comes from Rockford Fosgate’s ratings or from calculations based on them. The manual states the ratings follow CTA-2006 and are measured at 14.4 V. The CTA-2006 standard defines how in-vehicle amplifier power is measured, which keeps ratings at 14.4 V comparable between brands.

Mode	Rated RMS	Dynamic power	Total rated	Dynamic headroom (calculated)	Rated output voltage (calculated)
4 ohm stereo	50 W x 4	70 W x 4	200 W	1.5 dB	14.1 V RMS
2 ohm stereo	75 W x 4	115 W x 4	300 W	1.9 dB	12.2 V RMS
4 ohm bridged	150 W x 2	230 W x 2	300 W	1.9 dB	24.5 V RMS

Headroom is ten times the base-10 logarithm of dynamic power divided by rated power, and output voltage is the square root of power times load. Dropping from 4 ohm to 2 ohm loads raises the rating by 1.8 dB. That is a modest gain, so a 2 ohm load will not make the amplifier dramatically louder.

The other published figures are a frequency response of 20 Hz to 20 kHz, THD+N under 1.0 percent at both 4 ohms and 2 ohms, and a signal-to-noise ratio above 70 dB under CTA-2006. Rivals publish higher signal-to-noise numbers, such as Kicker’s above 95 dB A-weighted, but the reference conditions differ, so the numbers should not be compared directly. Rockford Fosgate also says its P.O.W.E.R. supply raises output as supply voltage rises, though it publishes no figures for that.

Installing the R2-300X4: wiring, fuse and fitment

Rockford Fosgate’s specification calls for a 60 A fuse (not included) and 4 AWG power wire. Some retailer pages suggest 8 AWG wire and a 50 A fuse, which conflicts with the manufacturer, so the manufacturer’s numbers are used here. The manual requires the fuse within 18 in of the battery, a ground lead under 30 in on bare metal (never a seat-belt bolt), and RCA cables routed away from power wiring.

Wire gauge matters more than it looks at the amplifier’s 39 A maximum draw. Using a 17 ft power run and a 3 ft ground, the length of the Rockford Fosgate RFK4X kit, a calculation with published copper resistance gives a drop of 0.19 V (1.4 percent of 13.8 V) in 4 AWG and 0.49 V (about 3.5 percent) in 8 AWG. The heavier wire wastes 7.6 W as heat against 19.1 W for the lighter one. The 39 A figure is a sine-wave maximum, so music-level losses are smaller, but the gap between the gauges stays.

Use this fitment checklist before buying:

• Electrical load: 39 A at 13.8 V is up to 538 W of input power, and the 60 A fuse sits 1.54 times above that draw. Rockford Fosgate says a healthy stock system should cope, though battery and alternator life can shorten slightly, and it suggests a heavy-duty battery or energy storage capacitor.
• Speaker load: the manual does not recommend loads below 2 ohms stereo or 4 ohms bridged. Two 4 ohm speakers wired in parallel on one channel present 2 ohms.
• Radio type: a factory radio without RCA outputs works through the Molex high-level plugs, and auto turn-on only functions on that route.
• Wiring kit: Rockford Fosgate’s RFK4X kit holds 17 ft of 4 AWG power wire, 3 ft of 4 AWG ground wire, 30 ft of 16 AWG speaker wire, one 16 ft RCA cable, 20 ft of remote wire and a Maxi fuse holder with a 100 A fuse. That fuse is above the amplifier’s 60 A rating, so confirm which fuse you install.
• Mixed wiring: Rockford Fosgate’s wiring diagrams show the rear pair bridged for a subwoofer, but it publishes no power rating for front stereo plus rear bridged use.

How the Rockford Fosgate Prime R2-300X4 compares with rivals

Three comparable four-channel amplifiers are the Kicker CXA300.4, the Kenwood eXcelon XR401-4 and the Pioneer GM-D8604. The Kicker is a Class AB design with similar 2 ohm and bridged ratings, while the Kenwood and Pioneer are Class D designs with higher ratings.

Specification	Prime R2-300X4	Kicker CXA300.4	Kenwood eXcelon XR401-4	Pioneer GM-D8604
Amplifier class	Class D	Class AB	Class D	Class FD
RMS per channel, 4 ohm	50 W	40 W	75 W	100 W
RMS per channel, 2 ohm	75 W	75 W	100 W	150 W
Bridged, 4 ohm	150 W x 2	150 W x 2	200 W x 2	300 W x 2
High-level inputs	Yes, 4-pin Molex	Yes	No, preamp inputs only	Yes, via supplied adapters
Crossover range	50 Hz to 250 Hz	50 Hz to 200 Hz	50 Hz to 200 Hz or 2.5 kHz to 10 kHz	40 Hz to 500 Hz
Height	1.9 in	2.4 in	1.4 in	2.4 in
Footprint (calculated)	68.0 sq in	89.4 sq in	60.8 sq in	86.6 sq in

The Rockford Fosgate matches the Kicker at 2 ohms and bridged but trails the other two on every power rating. It does offer high-level inputs, which the Kenwood lacks, and a narrower 50 Hz to 250 Hz crossover than the Pioneer. Kicker also lists a variable bass boost on its model, which the R2-300X4 does not have. Footprint figures come from retailer dimension listings for the Kenwood and Pioneer, and the Pioneer is the largest of the group.

Who should buy the R2-300X4 and who should skip it

Buy it if you are adding an amplifier to a factory radio, plan to power front and rear speakers rated for roughly 40 W to 75 W RMS, and want clip lights that make gain setting less of a guess. It also suits drivers who want a short, flat chassis and the option of a two-year warranty. Check the current Prime R2-300X4 listing on Amazon if that profile matches your plan.

Skip it if you want the most rated power, since the Kenwood and Pioneer publish higher ratings (competitor prices were not compared). Skip it if you need a bass boost or remote level knob for a subwoofer, or if your speakers need more than 50 W each at 4 ohms. Bridging gives 150 W, which is modest for a large subwoofer, and a dedicated mono amplifier from the same range is better suited to that job.

Rockford Fosgate Prime R2-300X4 price and pre-purchase checks

The R2-300X4 costs $309.99 USD. Prices change often, so check the current listing. At that price the rated output works out to $1.55 per rated watt with four channels at 4 ohms and $1.03 per rated watt at 2 ohms. Wiring, fuse and mounting hardware are extra.

• Model name: confirm the “R2-” prefix, because an older Prime R300X4 is a different amplifier with different specifications.
• Seller: Rockford Fosgate’s warranty covers only the original purchaser of a product bought from an authorized U.S. dealer, and the buyer must keep the receipt. Products bought outside the U.S. are covered by that country’s distributor.
• Warranty length: Prime amplifiers carry one year, and a matching Rockford Fosgate installation kit adds a second year. The extension needs the kit’s UPC code and the amplifier’s original Performance Verification Certificate.
• Box contents: the manual lists the amplifier, mounting hardware, an Allen wrench, 4-pin Molex pigtail connectors and the manual.

If you shop on Amazon, confirm that the seller is an authorized Rockford Fosgate dealer before relying on the warranty, and review the R2-300X4 seller details on Amazon first.

Rockford Fosgate Prime R2-300X4 rating explained

Area	Weight	Score	Contribution
Performance and sound/function	40%	3.5	1.400
Features and usability	20%	4.0	0.800
Build quality and design	15%	3.5	0.525
Value at the review price	25%	3.0	0.750
Weighted result	100%	3.475, rounded to 3.5 out of 5

Our research score is based on the rated figures and documented details above. Performance is held to 3.5 because the ratings are sound but no independent measurements exist, and the published signal-to-noise figure is modest next to figures some rivals publish. Features earn 4.0 for the input options, clip lights and crossovers, offset by the missing bass boost. Build scores 3.5 for the low profile and protection circuits, with limited published detail on internals. Value is 3.0 because rivals publish higher power ratings and the price buys 50 W per channel at 4 ohms.

Sources and further reading

• Rockford Fosgate R2-300X4 product page: specifications, features and crossover details.
• Rockford Fosgate R2 installation and operation manual: rated power conditions, wiring, fuse, mounting and gain setup.
• Rockford Fosgate RFK4X installation kit page: kit contents and fuse size.
• Rockford Fosgate extended warranty article: warranty terms and the matching-kit extension.
• Sound of Tri-State product page: retailer price and retailer wiring suggestions.
• Kicker CXA300.4 product page: rated power, class, inputs, crossover and dimensions for that model.
• Kenwood eXcelon XR401-4 at Crutchfield: rated power, inputs, crossover and dimensions for that model.
• Pioneer GM-D8604 at Crutchfield: rated power, class, adapters and dimensions for that model.
• HyperPhysics AWG wire table: copper resistance values used in the voltage-drop calculation.
• CTA-2006-D standard page: how in-vehicle amplifier power is measured.