Good passive subwoofer?

I used to run two HSU 1220 tube type subs with an Adcom 555, and it worked fine.

BTW, I don't remember any way to bridge a 555, so I'd be careful about that. I don't think that's do-able.
You shouldn't need to bridge anyway, as it has suitable power for two subs, which would be the preferred method.

Alternately, as has already been suggested, flog the Adcom and get a powered sub - I've seen plenty of HSU, paradigm, PSB and others that are very musical and reasonably priced.

Hmmm, my 555 didn't have a switch for mono. Older model?

Also - as bignerd says, you'll want to play with phase, paradigm has a good crossover too, and they usually go for around $100.

Just a little to augment Bignerd's thoughts - I suspect you know this trick, but in case you don't - to find best sub location, temporarily put the sub at your seating position, then walk around the room and see where it sounds loudest - that's where you should put it.