I think car sub amps can be used in this situation - specially for home theater if distortion is there, and if it is a concern.
yep a car sub amp
a mono can probably be bought in 5-6k
but ive never done this before
maybe someone more experienced can throw light if this is possible and feasible for even listening to stereo music.
Edit : i think one of our members has already done soemthing like this
6k for passive is reasonable ( actaully quite good for the finish i can see over the internet
but asking 20k for the outboard amp!
someone should see if they can find a hald decent mono amp for the sub for another 6k and i thin it should be fair