I have an XP-60, and I must say that the decay grunge-defect is not very audible. I have to turn up the volume of my amplifier to max as the sound decays to hear it at all. Besides, it is related to the D/A-converters and not to the synth engine itself. It only appears when the total signal-level from the synth drops below a certain level; it does not appear at the end of every instrument decay while playing. Besides this defect, the sound quality is excellent. Many people say the 30/60/80/2080 have better overall sound-quality than the 50/1080.
I would suggest anyone interested in the XPs to go and listen to a 60 or 80, and decide if they can live with the defect. If you can, then the 60 and 80 are great synths. I don't think this defect is a big problem.
Stig
Proud XP-60 owner..