How do you know you're running out of amp power? Did you compare the Denon's channel gain for your M22s and QS8s at your old vs. new place? If for example at your old place they were at 0dB and now they're at +3dB, it means your amp has to supply twice as much power to maintain reference SPL - at least that's how my Onk works because I've tested it. Even at +3dB, that Denon should have enough power unless your room is over-damped. How far from the fronts are you, how big is the room and what is on the floor, ceiling and wall surfaces? Are you able to take any voltage and current readings at the speaker terminals to calculate the power? If you can at least measure voltage using a 200Hz tone you can figure out the power from the impedance curve.
I see a gap in the market just like you. I'd like to buy an Onk (because I trust Onks) pre/pro only with all the same bells and whistles as one of their receivers. They can charge me the same price as a receiver so long as they employ a high fidelity design.
As for the 3-channel amp question, it's really a matter of whether or not you'll hit power limits on the surrounds if you power them from a receiver. I doubt my surrounds see more than 15W peaks on the most dynamic movie scenes. The center is a different story. As I've reported, my center sees 100W+ peaks.
