Good passive subwoofer?

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)

Thanks to all for the very helpful info. I looked at the VMPS website; it appears not to have been updated since 2002. Their phone number still works, so I left a message.
I will check out the other suggested brands as well.

How important is the phase control? What changes do you hear as you adjust from 0 to 180 (or whatever), and how noticeable are those changes? Can the same changes be achieved by adjusting sub placement?

Oh, another question. Is phase control more important with one sub or two subs, or is it equally important with both?

http://www.audiocircle.com/circles/viewforum.php?f=39&sid=07cac4d51643f2e4f8299ffef0ca7298

This is the VMPS forum, and there are two dealers here that hang out and can provide discounts. The European website is better, as the US one is pretty static.

http://www.vmpseurope.com/

Yes Phase control is important for both speaker for matching with your main speakers. I haven't had to use it as I've been pretty lucky but both subs would need that adjustment and is included on almost all crossovers.