Hugh -
I dont know if this will answer your question without going into technical detail, but this is what I experienced with the stock feet. Flick your finger on any of the feet while the turntable is playing on a quiet passage. Did you hear the thump go up into the plinth, into the arm, and hear it come out of the speaker? If not, flick harder.
The standard stock feet does not do a good job at isolating the vibration that can come up into the feet and transfering it into the TT. If you play the TT at low volumes, then you can probably get away with vibration not making it back to the TT - (not to impede on the work you did to isolate the TT from external vibration).
If you have the funding - upgrade the feet to Isonone. When you perform the "flick test", what will you hear? Nothing. Nada. I was very impressed!!! While you are at it, to help isolate vibration even futher (especially for those, like myself, who did not go through the work that you did to isolate the TT from external vibrations from the floor through the TT) you can get the rubber Isonone boots that are sold seperately which attach to the bottom of the feet for even additional vibration damping.
I have not used sorbothane feet, but I imagine it should do a similar job if you perform the "finger flick test". However, the Isonone looks way cool!!!
Again, I am not sure if I answered your question, but if you can stop vibration from coming through the feet into the TT, then I feel it is a step in the right direction. Now as to elimiate external vibrations getting into the plinth and the tonearm... I deffinetly do not have an answer to that - unless you can make the plinth and tonearm entirely out of rubber.
Mike
Anyone who thinks CDs sound better needs to listen to my system. Besides, my cables sound better than yours.
Technics SL1200-MK2 SE-1: MN Bearing + Baseplate + AL/CU Platter, Paul Hynes SR7 Power Supply, Isonoe Feet, SME V, Audio Technica AT33EV, CableMaker 1 Tonearm Cable
Technics SL1200-MK2 SE-2: MN Bearing + Baseplate, Copper Mat, Paul Hynes SR7 Power Supply, Isonoe Feet, SME 309, Audio Technica ATOC9/MLII, CableMaker 1 Tonearm Cable