I agree, as some Yamaha's tend not to collapse to mono very well, where as Roland seems to do a better job, and therefore may not benefit as much with two systems.

I did try a buddy's Roland G1000 through my system, through a single, and then both, and we agreed the sound had much more stereo spread with the latter setup.

As with the Yamaha, the stereo spread is heard nearly everywhere in the room...I don't really know why, as I'm no expert on acoustics, but I do know what my ears tell me...perhaps someone with detailed knowledge of how array systems work, could explain why this happens.

Not sure how the G70 or Korg PA-series would do...I can't see them sounding worse through two systems, although maybe the benefits won't be as dramatic.

Even my inexpensive Yamaha P-85 digital piano sounds way better through two systems...again, I don't know how other brands would fare...maybe someone else will have a chance to make some tests/comparisons.

Ian