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!
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.