We have had customers use our woofers in a ceiling-mounted, infinite baffle configuration. And the HX800 can power up to 6 woofers, 3 woofers per channel. They are used in the 6-pack sub towers so you dont need 4 amps to run them.
But as far as using them in the ceiling goes, the amps need to be close to the woofers, and I would also check local relulations in your area, as they may not allow for amplifiers to be used in the attic.
If you get the amps more that 3-4 feet away from the woofers, you start losing servo control as the signal from the sensing coils becomes too weak, and using repeaters would only add delay the signal.