Road Runner touched on this a bit on this already, but you guys really shouldn't get too hung up on the All-Channels driven specification - I know a lot of spec-jockeys are really keen on this figure still, but it's a pretty unrealistic way of comparing the real world performance of a receiver's power supply. It's another clever marketing trick some companies use, a different approach than the high distortion, low impedance power ratings, but misleading all the same.

You will find very, very few movies where maximum gain to all channels is sent consistently, for an extended period of time that equates to the all-channels driven tests. Instead, it's usually a blend of most of the gain going to the front 3 speakers in short bursts (allowing for recharge in between) and less gain going to the surrounds when applied. THX has done extensive testing on movie soundtrack patterns, and the most they'll check for in their standards is to see if an amp can sustain the voltage swing required to drive a few channels simultaneously for just a few seconds. Never 5 or 7 channels at max for more than a few seconds. In many cases, a an amplifier will deliver more power to 2 or 3 channels (ie in stereo mode) than another amp, but then fail to deliver the same power with all 7 channels driven. Which amp is more powerful? The one that gives more maximum output in real world (movie, stereo music) scenarios, or the one that can play a sine wave a bit louder in a laboratory with all channels driven?

The big Onkyo 805 has a healthy 875 watt power supply. That's the most that thing can suck from the wall without frying. That Rotel has a respectable 450 watt power supply unit. This should tell you all you need to know about which has more real world power capability.

That doesn't speak to the sound quality of the amplifiers themselves though, only their ability to not further distort the sound quality they are capable of achieving at high levels.

As a receiver solution, I would think the 805 would be sufficient for the 18 x 22 room size, though perhaps the lower sensitivity of the B&W's might be problematic.