I find the Tyros2/3 also sum to mono less than satisfactory...actually, no different than the S900.

I believe the PSR-3000 will have the same issue.

When we used the Roland G-1000 through one Bose, it didn't sound a lot thinner like the Yamaha did...but, it did sound quite a bit better with two Bose...my buddy and I both noticed the difference, altough it wasn't as dramatic a change going from mono to stereo as with the Yamaha.

I have heard about the poor summed to mono from several other Yamaha users that use one amp/speaker...so, they are obviously designed for use in stereo.

If one Bose system will do the job for your keyboard, and you are very satisfied, then you certainly don't need two of them.

Ian