woofersect.com may have a better selection. Hifonics is a decent brand for amps. You could also look at shalow mount or infinite baffle subs. A pair of dual coil 2 ohm or single 2 ohm would work nicely with most amps. If you can find one that has level line inputs which will let you tap into the rear left and right channel to power the amp or get a good Line Out Converter (LOC). I used T taps in my E46 to wire an LOC in the trunk from the OEM amp harness. That way I don't have to splice or cut any wires. I've been thinking of upgrading my front speakers with a good set of components up front and placing 6 inch subs in the rear door midrange locations : )
Check these out, they may fit better under the seats
6" Subwoofers