Alpine S2-DB8V-TRK 8-inch Ported Dual Subwoofer Enclosure Custom-fit Audio Packages the HDP-D90 includes an additional

the HDP-D90 includes an additional 2 input channels via a pair of RCA jacks

It's large

SAT radio antenna adapter

it will also fit where traditional amplifiers cannot

Alpine S2-DB8V-TRK 8-inch Ported Dual Subwoofer Enclosure Custom-fit Audio Packages the HDP-D90 includes an additionalProduct highlights: 8" ported dual subwoofer enclosure adjustable rubber feet for uneven surfaces Pro Link design allows width adjustment for secure fit loaded with two Alpine S2 W8D2 subwoofers Kevlar reinforced pulp cone with Santoprene rubber surround power handling: 600 watts RMS 2 ohm impedance frequency response: 35 215 Hz sensitivity: 84 dB dimensions: 51" W (max) 40" W (min) x 8. 75" H x 13. 75" D