If you're into DIY you could try a few of the UCD modules. They're supposedly good with low impedance loads, such as a sub. The multichannel powersupply is $195 which powers up to 5 of the ucd400's which are 400 watts into 4ohms, 200 into 8 and cost about $150 a pop. Then you'd just need to add modules as needed, for the buttkickers you could probably even use the ucd 180's which are 180 into 4ohms, and cost under $100 each module. So say, 2 ucd 400s, multichannel ps, 2 ucd 180s, and a transformer, plus have the room for one more channel down the road for about $800. Not a bad option, might be a bit more than you want to spend though, what's your budget?