Rega P3-24 help required.Right Ch. Distortion

Have you checked out the incognito wire for your tone arm? It will take your Rega to a new level.

I need to work around the marble - it is the 'top' on the cabinet - can't ditch it. Many users have employed heavy, dense materials in use for isolation - granite bases for TT, speakers, amps etc. Why not marble?

Miner,
I used to have a Rega P-3, and was able to effectively isolate it on the cheap using sorbothane feet. Starting with a level surface, fine tuning the level was just a matter how I placed the feet in relationship with the turntables weight/balance. I've also had excellent results with a Wilson Benesch table and the very affordable Foculpods. Once again, placement of said feet allowed for fine tuning of the tables level. The table sounded great, and low frequency oscillation was never a prolem. Not exectly a Vibraplane, but for less than $40, it worked awesome.
Currently I'm back to using my VPI Scout, and if I chose to isolate it further, I must use an isolation platform due to the separate motor assembly. I've previously used a granite plinth under my table, and never had any issues with ultra low frequency oscillation. Perhaps this is due more to the function of the unipivot arm, as opposed to a gimbled arm like the Rega.
Of course taking the time to properly setup your table will pay huge dividends, sound wise. A website that I've found helpful is vacuumstate.com. Check out the white paper by Allen Wright for some great advice, and a free alignment protractor. (if you have a printer)
Good luck!