Good people, sorry to come in late, hope it's better than never.
Marco, that's a beauty. Mine looks similar except currently sporting G707. Had a Black Widow for awhile back in the late 70s. There was a tonearm for which the right cartridge was never designed...
Anyway. One experiment I tried was to fill the subchassis (from the bottom) with melted parafin (the jelly-canning kind). This changed the overall bounce and maybe sucked up a bit of the vibration, I don't know. For awhile, about the last five years, I was kind of happy with the bounce. Then the belt broke, so I bought a new belt from Turntable Basics. My impression of this new belt is that it is a bit too small / tight / impedes the bounce overmuch. Nice belt, though, otherwise
Another thing with mine - the steel top plate is not exactly flat. This may affect the verticality of the spring posts, which may affect the bounce.
I have read - somewhere - that if you use a screwdriver to turn the spring posts on the chassis, you can modify the bounce. Perhaps! that would imply that they are not exactly straight...
Some years ago I took the foam crud out from around the spring posts. Did that help? Don't know.
You can make a new armboard (not as pretty, but that funky Scots pressboard is, well, funky) from MDF. An interesting experiment. My Grace was originally jacked up a bit on its pillar so I could use a thicker piece of MDF - 1/2" if memory serves worked OK. Better? I dunno.
Your clutch issue - haven't had that problem myself. However, mine is worse, I think - my motor shaft is slightly eccentric at the pressure pad end and the motor produces a bit of 300rpm noise above and beyond the 60hz noise.
I may one day replace my old Grace with a Rega. Cartridges these days seem to like being held firmly - who doesn't - more than back in the old V15-III days, though as I recall my 'table had a Supex 900 when it was in the store window. That must not have been a match made in heaven - vestigal tonearm and completely non-compliant cartridge. After my V15, I had an FR1-3F for several years. That was not a good match either. Latterly it has had a Linn Adikt which still throws the Grace around too much for my liking.
Good luck. A beautiful rig, that's for sure.