6" or 8" subwoofer for behind seat

[cochino12]

I'd go with the 8", you will find a better selection of woofers and they are very affordable. An enclosure for an 8" sub is probably going to be .3-.4 cu ft which is pretty small and with a quality sub you should be able get it to play down to ~30hz which sounds like it should be plenty low for your sound preference.

I'm a big fan of JL audio, the entry level stuff is pretty cheap and has always lasted me for years.