Which is the best Subwoofer?

D2 is dual voice coil 2 ohm(2 ohm per coil) and same with D4. I have 2 of the TS-W308D2's hooked to a kenwood KAC-9104D and those hit real hard. So I would definately recommend a Kenwood KAC-9104D Mono amp to power tham. The 307D2's and D4's are just the previous model of the 308's and probably wouldnt sound much different.