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.

Even if you will be using a very good pre-pro you may want to invest in a subwoofer crossover so that you can play with things like phase. NHT has one that is not very expensive and should work well.

Or you can sell the Adcom and get a good powered sub as mentioned above.

Good luck!

I use my old Adcom GFA-555 to drive the woofers of NHT ST-4 speakers with very good results. The Adcom is bridgable by flicking the little switch on the back, and it's bridged mono output is 400 watts @ 8 ohms, not 600 watts.

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.

Audio Control Richter Scale III has a bass eq, crossover and microphone (I use this in my HT on my Velodyne and used it on my VMPS)

Or http://www.behringer.com/DSP8024/index.cfm?lang=ENG

Or Tact system (which is awesome by the way but way more money)

On bridging the amp, get a VMPS larger and you can use a channel on each driver! but that sub is huge! (15" and 18" plus a passive radiator)