If you are looking for extra bass...
 

...without losing your boot to a big sub, then you could try one of these:

Alpine SWE-1000 8" Powered Subwoofer Box £110.99 Alpine, Sextons Direct

Ok, so it's not as low and loud as a proper amp and 12 inch sub in the boot, but it does go usefully lower than the 6.5 inch components you may have replaced the stock front speakers with. Best thing is it fits under the drivers seat (in my bugeye - can't comment on other models), although strangely not under the passenger seat. It's completely hidden and can't be seen from outside.

I'm running this off my Alpine headunit, but it should work with any headunit with pre-outs, which is most semi-decent units.

Just looked at the manual again - seems it will also run from speaker level outputs, so you can even run it from a headunit without pre-outs.

Hi escott,

do you know if this sub has an outward rca connection so that you can daisy chain a couple of these from one rca (from the head unit)?

Nope, don't think so.

You can just use a phono Y cable for that.