I can answer one question here that has to do with the platter and platter pad:
When the needle tracks the groove, the vinyl can talk back to the stylus and thus affect the sound. Just turn down the volume when you are playing an LP and you will see what I mean.
If the vinyl is properly damped, it will sound more neutral and correct, regardless of the drive system. This is because this is a more important feature than the drive. Sure, you can hear differences in the drive, not contesting that.
Anyway, most platter pads don't damp the LP properly! Acrylic is too hard as are metal platters (which work fine if the right platter pad is in place). Rubber and cork are too soft. Warren Gehl of ARC did a lot of reserach in this topic about 20-25 years ago and over a period of years developed what was and still is the best platter pad I've seen. It has the same hardness as the vinyl, which is paramount as if it is any different, the coupling between the two surfaces will not favor all frequencies. At the same time the pad did not depress as the needle tracked the vinyl (there is a microscopic depression around the needle caused by the tracking pressure), yet the pad was designed to damp not only the LP but also the platter.
Sadly, the last of these platter pads was made a good 15 years ago or more, but what those audiophiles that were lucky enough to find one found out is that it almost didn't matter what table you had if it could support this pad (which weighed a few pounds). They all sounded pretty much the same if the pad was in place, which is also to say considerably better than without!
I think someone needs to make a new version of this pad. Warren says he can no longer make it as some of the materials aren't available.
BTW I've played master tapes against the resulting LP many times. I'm not sure I would use the term 'overdamp' but I know what you are talking about. This is actually a coloration caused by the platter pad. Raul is correct- it is actually impossible to in reality overdamp a turntable (or a tube or a transistor...); if you hear something like that what has happened is a coloration has been introduced.
Please note that I did not say its impossible to overdamp a cartridge. With LOMC all that happens is you loose output, but with a moving magnet you will indeed kill the highs if the cartridge is overdamped (meaning the load impedance is too low for it to drive with full bandwidth).
