How to isolated stand from springy floors?

Interestingly, the OP makes no mention of problems (skipping, feedback) with the turntable; in fact, he suggested that there is no issue with the sound. The turntable, while it has a type of suspension (rubber feet between the chassis and plinth) and is generally very good, is not known for being particularly immune to this sort of problem. If the floor were truly "springy" this would affect turntable playback. Before undertaking more drastic fixes, I would look for an extremely microphonic tube in the amplifier; replacing each tube one at a time to isolate the culprit(s) and also putting some isolation device under the amp. I would suggest that the problem is not floor springiness but a tube or an amp that does not have good immunity to low frequency excitation; something that would probably not be solved by adding mass underneath it and, in fact, could make the problem worse.

04-09-14: Czarivey
Usually bad tubes are more sensitive to mechanical disturbance.

04-11-14: Frogman
Before undertaking more drastic fixes, I would look for an extremely microphonic tube in the amplifier; replacing each tube one at a time to isolate the culprit(s) and also putting some isolation device under the amp.

Among the many comments that have been made I particularly second these. I'll add that another way of determining if a tube is excessively microphonic is by LIGHTLY tapping each tube with a pencil eraser, while the amp is powered up but no music is playing, and seeing (i.e., hearing) what happens.

Regards,
-- Al

Thanks for all the advice, I have been trying out few different variations and I think I might have this worked out.

Since I posted this I have changed my TT to a Garrard 301 with a very heavy plinth and I also purchased some of Herbie's big dot gliders to go under the whole rack. I think between the extra weight on top of the rack and the dampening feet I have found a pretty good balance. It could have been that my rack was simply just a bit to light in the beginning and the extra mass has fixed this issue.

After my success with the initial change I got some of herbie's SS dampening rings for my pre tubes and I still haven't really decided if I like the way they changed the sound. They give everything a bit of a thinner feel and have removed a bit of the romantic/lush sound my system used to project. That all said they do do pretty much exactly what they claim; tighter bass, cleaner highs, and a bit of a sound stage bump.