I must agree with Fran--it's something that clearly seems to be unique with Yamaha's stereo sampling technique and in the way it is applied. And, it primarily seems to be associated with the piano sounds. Additionally, it was not present with some of the older keyboards, many of which had incredible piano sounds such as the PSR-5700, which I ran through the PAS just this past week.
As for the KP-100s, I have one for a backup just in case the PAS or Z-5500 systems fail on the job. I keep it hidden in the van all the time, and in an emergency it can get through the job. However, it is not my amp of choice for anything--it sounds awful at best--even when the EQs are tuned. And, while the sounds will improve somewhat by swapping the speakers out for higher quality speakers, the sound is still sub standard IMO. I also tried the KP-200s a few years ago, and while it was somewhat better, it wasn't even close to the sound quality of the Peavey system that I was getting rid of.
The bottom line--I'll stick with the Bose PAS until something better comes along.
Good thread!

Gary

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Travlin' Easy