I might have missed something here.
If surround modes suck with the Emotiva amp, why would adding more Emotiva amp modules improve the surround modes?
It seems like the surround modes sounded great WITHOUT the Emotiva. So how about just don't use the Emotiva amp?
Oh, wait, did you mean that you think using a 7Ch external amp will improve all the sound? This way, you don't have to use the Yamaha AVR internal amps at all? So all 7 speakers will be powered by an external amp?