Mike:

Good test and some nice music too. Thanks for trying to provide an objective demonstration of the phase cancellation phenomena. I honestly am not convinced that this is a fault of Yamaha or Korg, or at least it may not be something that they accounted for. Phase cancellation happens when two signals meet each other that are out of phase. It's easy to make this happen and Korg and Yamaha probably didn't think that a lot of people would listen to their PA1XPro, Tyros or P250 or any of their stereo instruments in summed mono... that would be like blaming Sony if your home stereo didn't sound good in mono - they probably didn't think you'd use it that way. Some keyboard manufacturers have accounted for this and provided a mono output (and mono samples?), some haven't. I don't think there's anything that can be done about it, at least until newer models come out and only if customer demand for a mono output forces the issue.

Here's a good quote from UD: "If your kb sounds like crap in mono ... it will sound like crap, only louder in the PAS." Let's say we buy that theory because it may well be correct. That means that you really have to shop for a mono-compatible keyboard if you plan on going with just one of the PAS units, unless you find phase-cancelled sound acceptable. If you go with stereo then you are free to choose any keyboard on the market. Hmmm, this could be an interesting discussion after all...