So I actually bought Stillpoints and their HC Mini Inverse Risers, plus 2" diameter Sorbothane Hemispheres. I placed the Stillpoints under the granite shelf and did my basic tapping test on the rack and the granite, and back and forth. I also used the Revox at maximum input to assert the findings.
The Stillpoints are extremely effective in reducing feedback, to the tune of about 20dB, and as you can tell, quite audible. Then I placed the sorbothane feet under the VPI's feet - yes, actually the pointy feet sink into the sorbothane, quite a bit in fact (pics will be provided tomorrow). Tapping test again on the rack and granite - also extremely effective in reducing feedback by about another 20dB. That's probably not surprising. But when I compare these results to what the HRS S1 isolation platform gave me - at $1000 more - which was about 30dB total reduction, I was stunned to have been able to find such an effective isolation at a much lower cost.
OK obvious question - how can I be so sure there was this kind of difference? The Revox did the job: with the HRS, after tapping there was residual subsonic energy clearly shown in the VU meters, which took about 1-2 seconds to dissipate! With my tweaks, no such energy, no matter how hard I tapped. Consider this: with the HRS, the VU meters first go sky high, then drop, then back up again (no audible sound at this point), and finally oscillating as they eventually slowly go silent. With my tweak, pure linear travel - up and then quite fast down to rest, with no overhang. Really fun to watch.
When I moved on to the listening tests, the audible effect of these two tweaks wasn't any different than what I achieved with the HRS platform. Surprising??? Probably not - the granite is a 50lb shelf much like the HRS, and the tweaks are just as effective as the HRS's rubber-like (polymer?) feet. The 'table is also very stable because the feet sink very deep into the Sorbothane. Will the Sorbothane eventually survive the VPI's weight? Unknown yet, but this is a configuration worth keeping for now.
The audible improvement is as you would expect - gone is any bass overhang, rendering clear and tight bass drums, and any ringing of the granite and any motor-induced noise is largely absorbed by the Sorbothane, so the sound is very crisp - frighteningly so in fact, especially with drums... Next up is actually more Sorbothane, this time as thin sheets under the motor, for which I will design some sort of mini platform to sit on, with the Sorbothane sheet underneath it. Stay tuned.
(The Sorbothane hemispheres are a mere $17 for four -
http://www.amazon.com/Sorbothane-Rubber-Bumper-Non-Skid-Feet/dp/B003IMODW8/ref=pd_rhf_p_t_1)
