Best rack for non-suspension turntable?

A more cost-effective solution than the Halcyonics, with comparable performance, is a
Minus-k BM-8 isolation system ($2500). My recollection is that the WTC is not a high-mass design (< 100 lbs) and therefore well within the payload range of the BM-8. I've had one for about 6 months and have been enormously impressed with this isolation system and the improvements it has wrought from my VPI TNT.

Be aware that using multiple layers of isolation materials can have a negative effect on vibration isolation. For example, if you have two platforms and two layers of isolation material, this is equivalent to a two-mass/two-spring system of coupled oscillators. This system has two resonant frequencies instead of one (so-called normal modes or eigenmodes). One of these normal modes will have a natural frequency higher than that of the equivalent one-mass/one-spring system and the other will be of lower frequency. Because the natural frequencies follow a sort of fractional power-law dependence on the masses and spring constants, the eigenfrequencies are not far (within an order of magnitude) of the value for the one-mass/one-spring combination. All other things being equal, this is not what you want because the goal of effective vibration control is to keep the natural frequency of the isolator well below that of the system that you are trying to isolate.

I have gotten excellent performance from Minus-k products both in my laboratory and in my home audio system.

Every material has a set of resonant frequencies (mode spectrum). I believe you may be referring to the boundary conditions that apply to the problem and this is why I qualified my comment with the phrase "All other things being equal, . . . ." The example of a partially buried telephone pole shows how boundary conditions affect resonance - one end of the pole is clamped, which shortens the effective length of the pole and raises the resonant frequency.

Layers of adhesive or semi-adhesive polymers like Sorbothane also have resonant behavior; the natural frequency depends on the size and shape of the layer. The product web site has data and a nice downloadable calculator to compute resonant frequencies for various applications. On looking into this, I was interested to learn that it's pretty difficult to get the material's resonant frequency much below 5 Hz in any real-world application, especially for thin layers covering large areas.

I'd also like to point out that the Minus-k products contain no servos; they are completely passive. The principle behind these products is to use a coupled spring-flexure system in which some of the springs are near their buckling points. This can create a negative-stiffness situation, hence "minus-k."