Good passive subwoofer?

I have an unused Adcom GFA-555 amplifier rated at 600 watts bridged mono into 8 ohms. I am thinking about using it to power a passive sub. Does that sound like a good idea? It would be mostly for use with movies, as I am happy with the full-range bass in my KEF 104.2's for 2 channel music. Any suggestions for a passive sub? The signal path would be from the sub pre-out on my pre/pro or receiver (which I haven't chosen yet). Thanks!
VMPS has a very good reputation. Or, DIY.
I believe you would probably be better off passing the Adcom on to someone here on Audiogon and using the funds to invest in a powered sub. For example, HSU makes excellent subs for the money that would probably fit your ticket and give you more flexibility than a passive.
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.
I support the raw sub (cabinet with driver/s and inputs; no passive crossover built in) in conjunction with an external active crossover and external amplifier. This provides much more flexibility as well as allowing one to place the electronics on the shelf/rack with the rest of the gear.

Mounting electronics inside of a sub has to be about the worst thing you can do. Unless a fairly elaborate isolation/suspension is used to isolate the electronics most eventually fail due to the vibration of the sub. The only sub I've had for more than a year that has not failed, so far, is an old REL Strata. Mirage, Energy, Sunfire, JM Lab; they have all failed; usually just a minor part or loose wire as a result of vibration.

A pair of subs with smaller drivers (say, dual 8" drivers in a push/pull arrangement) would probably be best for the most musical with 10" drivers being the best compromise between "quick" drivers and deep bass. You then run them in stereo which helps with a lot of placement issues.

The Bryston active crossover appears about the best value in a quality crossover but the Marchands are attractive, too, if one wants a tubed unit. There is never any shortage of powerful used stereo solid state amps so that is a no brainer.

Tyler Acoustics will build custom subs; already checked that out.
I'm using VMPS in my hometheater for around 8 years now...stunning bass. Two Supertower/R's (the lower section of these are the large VMPS subs)..and one large VMPS sub in the rear of the room.

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.
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.
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!
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 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?

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.

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.
Phase shifts can make a big difference. Without a sub crosover you can shift phase 180 degrees by inverting the polarity at the speaker or amp. Speaker placement is still the most important consideration though.

If your room is symetrical and you can place the subs symetricaly in the room (and still move them about for optimal performance) then stereo subs is the way to go. Most sub controlers are going to shift phase and change levels equaly to both subs.

If your room is not symetrical (or you can't place them symetricaly) you will be better off with one sub placed properly. Plus one sub is about ten times easier to set-up than two.

Good powered subwoofers are easy to set up and sound very good. It helps tremendously to have a patient helper to adjust the controls for you while you sit in the sweet spot.

Good luck!
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.