I've got a pair of studio monitors which I'm very happy with, and am considering matching them with a sub. Since I can only hear the sub in the store, and without the benefit of hearing how they'd sound with my current monitors, what can I do to make sure that they will sound good together?
To do this properly, you will need the frequency response curves for both your studio monitors and your subwoofer so that you can get the crossover frequency right:
In the above graph, the red curved line is the subwoofer frequency response. The green line is the frequency response of your studio monitors. The crossover frequency is the point where the two lines cross.
In an ideal world, the point where they cross is -3db down for both.
Guess you should look around through different subwoofers on the internet and read their reviews, once you have the subwoofer you want you can learn about where you need to place your subwoofer.
Most studio monitor manufacturers will make subwoofers that match.
Also, many active subwoofers will let you connect your speakers via the subwoofer, via it's own internal cutoff-filter, so you don't get an overlap in output. However, you can still get a mismatch if you instead of an overlap get a gap.
I have a subwoofer in my home setup; however I have moved away from using it to monitor whilst mixing. If you speak to experienced mix engineers, they will ALWAYS recommend that instead of buying a subwoofer, you sort out your mix position with acoustic treatment - you can do a lot to improve the overall response of your room by installing bass traps (the more the merrier!) and diffusion between and above the mix position. Bass nodes are a massive problem in any room; you'll hear it as a 'lumpy' bass response (or excessively boomy at particular frequencies, in a pattern). As a result, you end up cranking the bass more and more to compensate - or EQing the bass in a weird way which actually sounds awful when you listen on another soundsystem.
If you treat your room well, you can find you can then hear the actual low frequencies - which your nearfield monitors are cranking out! - but which were previously hidden by your room's suboptimal response. It didn't stop me buying a sub as much for listening pleasure as critical mixing :-)
My advice is, generally, buy your sub - any decent one will do, just get one a sweepable crossover, gain and phase adjustments AND A BYPASS FOOTSWITCH! Put the sub in the mix position (where you sit) and adjust it so the rolloff isn't impinging upon your nearfields' rolloff. Then, get on your hands and knees and crawl around the room whilst listening to reference audio! When you find the position where the bass sounds the most natural and consistent, place the subwoofer there. It does mean you have to compromise your room wiring and equipment setup, but that's what you need to do to achieve as consistent a sound as posible (without resorting to loads of bass traps all over the place).
The BBC use dual subwoofer setups in some mix studios, when I was in their Birmingham mix studio for The Archers drama serial a few years back I noted they had dual Genelec subs in opposite corners of the room. However, remember - the more subs you add, the more complexity you add and the more you compromise your room acoustics (so many room nodes!)