I use an XP-60 and it works great, except for one thing. Lately one of the A flats has become hypersensitive. If you hit it soft, it plays loud. If you hit it hard, it plays LOUD. It is totally out of wack with the other keys. I've been wondering if something was up with the velocity anyway, because recently the PianoStrings sound (which I use frequently when playing live) has been getting harder and harder.

Any brilliant insights or experience with this type of problem?

Unless it's the key(s) going bad, I can only come up with that you've been messing around with some settings. I know some synths have settings for full velocity, no matter how gently you play. Maybe it's some OS bug.

-tek