bpoletti,
I can't give you any solid knowledge on this issue. Even when I tried to get info, suggestions were all over the place. You can't do one thing that another audiophile will telly you is Absolutely The Wrong Way To Go!So take whatever I contribute with a grain of salt.
I went down the rabbit hole "researching" (on a layman's level) many isolation materials and the various footers, isolators etc.
I used a siesmometer app on my ipad, to get objective read outs of vibration isolation, and used the "hand test" (what I could feel coming through and other off the cuff techniques as well. So...not exactly lab work. But the best I could do given the wilderness of contrasting opinions.
I ended up with a 2 1/2" thick maple block, atop various materials but most importantly:
The single most effective isolators were spring based - in my case Townsend Isolation Pods. They measured, and felt, vastly more effective than any other material or footer I tried (sorbothane, vibrapods, Isoacoustics pucks, etc).
That is for stopping major vibrations getting TOO the turntable on top of a block held upon those springs. Stamp the floor around the turntable and you can feel virtually nothing, and the seismometer app registers very small readings vs the huge spikes without the springs.
But...putting the springs beneath can it seems add a tiny bit more ring-off when the turntable itself is tapped (thinking of turntable born-resonance, rumble here, possibly being slightly amplified by being placed on springs).
So...the best approach I could come up with is: use springs, but holding up fairly heavy materials above. If the object held by the springs is too light then you get some feedback if the turntable is pinged sitting on the springs. But the more weight you place between the turntable and the springs. the lower the feedback to the springs.
Once you have a heavy base atop the springs it seems to be the best of both worlds: tap or stomp beneath the bass and the springs do a great job of isolating vibration to the turntable above them. But tap the turntable or the maple block above the springs, and that too produces even less vibration (measurable with the ipad app) than the turntable sitting on just a shelf or on just the maple block.
I've read some posts by some folks who seem quite experienced with these techniques who came to the same conclusion: best is springs incorporated into the system, but make sure they are holding up something heavy enough to also dampen any major ringing feedback from the springs.
Again...take all that for what it's worth: just another audiophile who tried to answer these same questions. I'm quite happy with the way my base turned out.