From my testing with the G70, personally, I hear no difference between using the mono out on the G70, and summing to mono externally. IMO, there's no need for additional gear (a mixer or Y-cord) when faced with a mono input.

And, to be honest, I am STILL waiting for anyone to show this on Yamaha's in any empirical sense. You have to be VERY careful that summing doesn't alter the volume, or tone differences WILL be heard... even though they aren't really there. The ear is VERY sensitive to changes in volume, but tends to percieve them as changes in EQ...

I have to admit, I'm also not a fan of big 'smile' EQ's. Studios spend a fortune mixing and mastering on speakers that are as flat as is humanly possible. Then we go any f*ck it up with a big smile, and then wonder why the vocals get lost, or the sound is brittle, or the bass is spongy... Me, I'd START with everything flat, and only adjust if you HAVE to. Bose's are pretty 'smiley' already, IMO...