307,308 are better cause , it have dual voice coil
which helps you in the connectivity from the amplifer
by means of resistance
either 1ohms , 2ohms , 4 ohms or 8ohms
these DVC sub are more beneficial for the selection of load for your amplifer ,(i.e how much u want ur amplifer should work on running sub , the more tuff ur amplifer will be , the less ohms u use for excellent result & max performance
306c 1000wmax i guess its 400wrms
307d2/d4 is newer then 306c 1200wmax , 400wrms
308d2/d4 is latest 1400wmax , 400wrms