What size subwoofer to get?

I'm using a Rel Strata III with Magnepans with excellent
results. The Rel is very tuneful, with a highly adaptable
crossover network. If will go down to 18Hz flat - how
much music is below that? I also agree that two smaller
subwoofers is usually better than one larger unit.

System-matching and your music-HT bias are critical. I use a Paradigm Servo-15 to supplement (not crossover) my B&W Nautilus 802's for the sub-50hz information on some (not all) music listening. The Servo-15 is an excellent HT sub and a good music sub. However, it's still too slow for the fast 802s on some music. Several knowledgeable "golden ear" audiophiles I respect have independently told me that there is nothing better than the Vandersteens for music (preferably using 2 or more). They each use 3 8" drivers and are very fast. I understand a cheaper alternative is NHT Sub Two's (2 10" woofers each), which are out of production. Though the Paradigm Servo-15 is an excellent compromise for music and HT, it just illustrates that you should be biased in favor of which is more important to you.

Peter, I would think having the additional ability to pick different roll off slopes would allow you to better blend the sub especially if you have a dip or a hump at or around the crossover frequency.

A question to Rives. I've been thinking of building two subs to be powered by a Bryston 4B in stereo mode for music and dual mono from the LFE output for HT. You mentioned that having the ability to vary the placement of two separate subs independently can avoid some of the limitations of only using a single sub. Is this assuming they will be reproducing identical mono signals? If the two are placed in radically different schemes how will this affect their ability to operate in a stereo set up? Thanks.

As I concur with the above advice I would suggets the Vandersteen Sub which used should run $750.It has an execllent crossover which is is accessed through a direct hook up to your speaker posts and responds to the full range signal not just what is low passed.Very clean and quick do also to fact that it uses multiple 8" drivers.Call a sub manufacturer and they will tell you that bigger is better but a large cone and magnet assembly does not always move fast enough to reproduce pitch accurately.If it were Home Theatre thast would be a different story where bigger is better for that "boom in the room".I vote for Vandersteen and if you can afford it the REL Strata III (starting with one and getting second).But all mentioned above are good.You didn't say what kind of assciated equipment you had so all of us are being presumptuous and as you will have noticed freaks.But as it was mentioned before a really cheapo sub good for HT will screw up decent sound.If you are looking try to find one with smaller faster cone and get two but also try to look for one with a servo amp which will produce less distortion than a non-servo.
G'luck

Gunbei: It's a good question, but fortunately you almost never need to compromise. The flatest response is with subs placed on opposite walls at the midpoint. Now if it were for music and it was a high crossover point (say 80Hz or higher) I would move them back closer to the main speakers but symetrical, and relatively close to the wall (within 1/8 of a wavelength of the crossover point). In a non-symetrical room, this may not be the case. Here's an even more interesting phenomenon (purely hypothetical--because I don't know anyone that's actually done a room like this). If you use 4 subwoofers and place them 1/4 of the crossover wavelength from the corners of the room you get a nearly flat response throughout the room--no peaks or nulls (obviously there would be minor ripples due to furniture and other items in the room in practice, but in theory it's a flat response). In this last example it does assume a mono signal. In your case, you would not need a mono signal because most of the lower bass notes are mono. If the crossover point is high--then that might not be true and you could run into some difficulties, but no more than having 2 main channel speakers that carry the bass information independently. So, I guess I don't really see any significant downside to stereo subs--I hope I've explained that adequately.