Good passive subwoofer?

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.

I'll second the VMPS, bargain bass that is tuneable. Just big boxes. Get a Pair for movies and use the adcom! I use to use a Hafler DH-500 with a pair of the VMPS standard subs and it was amazing for movies, they just take space (Wish I got a pair of largers!) Put them together(Screw the drivers in and cable it) and save some money too.

If you need a smaller sub and less bass, older NHT subs are passive and cheap, but are better at music versus Movies.

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)