If you're still using your head unit for amplification, you have no idea what you're missing! Head units are often unable to provide the loud, dynamic sound you want, and can distort (or even damage) your speakers at even medium volume levels.
That's where the BOSS RIOT Series of amplifiers comes in. Tried-and-true Class AB amplifier topology, 2-ohm stable (for powering a pair of 4-ohm speakers off a single channel), and way more than enough power for even the hungriest door, dash, or deck speaker. The R4004 can also be set up for dual 2.1 (dual tri-mode) use to power two pairs of stereo speakers and two mono subwoofers (passive crossover values are listed in the manual).
BOSS built the RIOT Series amplifiers with advanced features that are usually associated with amplifiers at more than twice the price: thermal and short circuit protection, soft turn-on circuit, remote turn-on/off, nickel-plated low-level and high-level inputs, LED power and protection indicators, all housed in a stealth black anodized aluminum chassis.
The R4004 also includes a MOSFET PWM power supply (for 4-ohm bridged-mono operation), dual variable 35 to 160 Hz low-pass crossovers, dual selectable fixed 80 Hz high-pass crossovers, a variable 0 to 18 dB bass boost, and a remote subwoofer level control for dialing in the perfect amount of bass from the driver's seat. Mounting hardware included.
Specifications: • Power output: 4 x 150W RMS (@ 4 ohms), 4 x 400W max (@ 2 ohms), 2 x 800W max (4 ohms bridged-mono) • Frequency response: 9 to 50,000 Hz • Signal-to-ratio: >102 dB • Total harmonic distortion: 0.01% @ RMS power output • Damping factor: >125 • Dimensions (L x H x W): 10.44" x 2.25" x 15.51".