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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.