Daniel's reply made me think of one. Someone mounted a Triad subwoofer in a wall at a client's home and, I kid you not, ran it off of the left channel of the power amp. Full frequency audio. No crossover.

I've recently bought an outboard Jamo subwoofer amp for about $300 to power that sub. there's a company not too far from me that specializes in speakers and such, and they carry many different amps. To use this amp, you'd have to agree that it had enough power -- how much did yours have? -- remove the amp from your box, plug up the hole with particle board in a way that did not leak air or let it vibrate, then use this amp externally. There is no appropriate way to put a different power amp into that box as sizes will be different and safety requirement are stringent for anything box-mounted.