You should demo around and see what you like, of course, but Polk offers some subs that don't cost a whole lot and fill a room with boomy bass. Paradigm has some subs in the low dollars range that are capable of the same. HSU and SV have cylindrically designed subs that go very low for a moderate price. If you bought a passive sub from one of those manufacturers, you could power it with any old amp you might have laying around or could pick up cheap to get reasonable performance.