I would think that would have alot to do with speaker efficiency. Having separate amps is generally better because of the physics of sharing a transformer under load, reserve power, damping factor, etc.. Unless maybe you buy a giant brute, like the 790.

Sounds like the monos are driving the left and right front channel. If so, likely its the center channel that needs it. Truely, most surrounds are running under 20 watts. The center channel carries a lot of the sound in a movie anyway, more than most think.

I would like to see Outlaw come out with a 7 channel amp that had 4x100 and 3x200 watts. Or possibly an 85 to 100 watt monoblock for $150 bucks. Then you could mix monos.