You can do alot w/900 watts

My complany has a single amp (Rockford Fosgate T3002) on 4 t215d4's(15"subs)
that puts out around 900 watts into a 2ohm mono load,in our scion and it will drive you out of the car, as far as a single sub make sure the sub has a high
SPL (sensitivity) plus I would NOT go buy the name or complany that makes the sub, I would look at the parameters such as spl,xmax,fs and so on
that is where you can tell if the sub is going to tell the man from the boys!!

If you need any help pm me!!