What I ended up with after many similar tests... in a solid brick house with suspended wooden floors (old style solid hardwood).
My Rack is made using several Ikea Lack side tables standing one on top of the other - the internal structure of these side tables, is hexagons of corrugated cardboard, it is very light weight, but is very efficient at damping/absorbing high frequencies - but does nothing at low frequencies. (the space between the lack table legs is precisely rack / component width)
At Floor level I have a large concrete paver, purchased from a garden store (I sealed it with wood oil, to ensure no concrete dust...)
Under the concrete paver, I have 5 of the largest size sorbothane pucks I could find
So heavy mass sitting on springs - very low resonant frequency - high absorbance of low frequency vibrations
An Ikea Lack Table top is then on top of the Paver, with a complete Lack Table on top of it - within this level I have a Power conditioner (glorified integrated power switch, but with some ability to absorb power spikes, and an insurance policy
) as well as my HTPC, and Power amp
Then I have another Ikea Lack Table, standing on some quite firm Sorbothane pucks - in this level I have the AVR, and the Phono Preamp.
On top of this Lack table I have the Turntable - the original feet have been removed and small firm silicone rubber ISO feet (5 of them) sit under the TT (the bottom board has a tendency to resonate - so the 5th foot in the middle made a substantial difference!
I also filled the TT plinth, and all available spaces that weren't needed for mechanicals or cooling, with plasticine... (oil modelling clay) - it is an excellent damper/absorber
I have a box hanging around with a wide range of different size silicone and sorbothane pucks and domes, also some magnetic ISO feet, spikes, vibrapods - I experimented with a lot of options - measuring them just as you are... (also did some walking and stomping around the room...)
Keep in mind that there are heaps of variables - and changing a component (eg: changing the Amp or AVR in the stack) would change the sprung mass of the entire rack - this in turn would change the rack's resonant frequency, which could bring it into a problem zone (or not).
A change such as this, could potentially require revisiting the various pucks and changing their springing constants (how firm/soft they are) for optimal results.... this is NOT a one size fits all situation - it is specific to YOUR equipment, YOUR cabinet, and YOUR floor/room.
But YES - remarkable results are achievable - and relatively economical turntables (new or vintage) can provide results that will shame many very very expensive TT's - simply through the benefits of isolation - a well set up isolated TT will always outperform a poorly isolated competitor, even if that competitor is much more expensive!!!
Of course, some such competitors, are expensive due to the extensive isolation and damping built into them... - but this sort of measurement can achieve equivalent results, at far more economical prices.... and it is an interesting and educational process... very rewarding too, when you hear the results!