Eh hem!...Subwoofers... What do ya know?

My main system is Apogee Duetta Sigs. They don't go down much below 40-50 Hz, so I added a Paradigm 15 Ref Servo sub - sounded good. Then I added a 2nd 15 Ref Servo sub and it sound great!. I have them wired in stereo and positioned as if they were part of the Apogees - in other words - right next to them. Works for me.

Subs for music vs Subs for HT. Big difference in what you trying to accomplish sound wise, IMHO.
I use a large woofer that is not that accurate for HT set up...and two REL T5i's for my big rig. The T5i's are very fast ( therefore they do NOT interfere with my main speakers..which is the goal) and are invisible SQ wise in the blend. What two subs do is exactly what REL states they do, which is to smooth out the response in your room. Unfortunately, setting up two subs is IME a ton more time consuming than setting up just one....but worth it in the long run.

richmos,

I had Duetta Signatures some years ago and they were really something special. They were very current-hungry and I ended up buying a Krell amp to feed them. They also liked lots of space behind them, but the sound was unmatched.

" How would you determine crossover frequency and gain with less than four subwoofers?"

Buying a sound meter is a good idea, sub woofer(s) or not.

You can determine when the bass starts to drop off from your mains, IN YOUR ROOM (not factory data sheet, that is 'roomless'.)

Use meter to find the best trouble free location of your mains first, then, get the answer when they start to lose bass, use a bit below that as a starting point for the sub crossover, then use your ears.

It's like an old carbureator, you have to adjust two factors (air/fuel mixture and idle speed) with each other, not separately.