Jtcf, and to others in general: I've used damping material (Dynamat) throughout the entirety of my CDP, including the underside of both the top of the case enclosure, and, the underside of the bottom of the enclosure. I can only say that the results have improvedparticularly with respect to higher frequencies. I first noticed it while hearing multiple glockenspiels play, then with other tuned and untuned percussion.
If adding vibration damping material to a CDP makes it sound worse, I should think the cause is either the CDP itself (e.g., by removing the vibration, you hear more clearly what the CDP is naturallywithout the additional influence of vibrationcapable of); or, the sonic signature introduced by the solution itself (i.e., in terms of what frequencies it may selectively dampen or not dampen well). Perhaps it's both, or, by damping vibration, you're now able to hear a problem that is quite different in nature. Whatever the case, I can't see it being what some like to call "overdamping".
I say this because it seems reasonable to me that, the only thing in a hi-fi that should vibrate are the loudspeaker cones. Anything else in the system could merely introduce signals that any other part of the system was never intended to introduce or entertainsignals that therein mask or distort the original audio signal. If, to the contrary, there are some circuits or electronics that must necessarily vibrate in order to perform optimally, I don't know what they'd be, and regardless, it doesn't appear that anyone is damping the electronics itself, anyway.