Marble or Granite shelfs in a hifi rack?

Flex, there's NO WAY that tightened phase, coherency and PRaT can be carried too far! I'm being a bit of a devil's advocate here for sake of lively discussion. You defend (it's ok) the Spectral as ALREADY having great coherence and PRaT, and that the Neuance only helps LESSER equipment. I beg to differ.
The Neuance can't possibly effect the electrical design excellence of an amp or processor, but it CAN keep out spurious and as well as corellated vibes. The PRaT/coherence continuum we aspire to in recorded music reproduction is indeed asymtotic, and not center (or other)-balanced! In other words live music doesn't have an "ideal" coherence or PRaT that can be exceeded by too much "coherent" componentry in its reproduction. You gan only get closer and closer with MORE PRaT-ful links in the chain, approaching the live event's infinite PRaT-fulness, if you will allow me the language stretch.
Perhaps you're confusing ambient acoustic issues with PRaT.
I certainly agree that a "live" musical event outdoors, for example, is way too dry because of absence of near reflections and far reflections (ambience and ideal reverberation decay envelope). Similarly a vocalist in a large stone cathedral with mics in the farfield is as "PRaT"-less as can be imagined. But that's not what we're talking about. The degree of reflected energy, and its timeliness, is decided by the recording engineer, and is locked into the "software". A really "PRaT"-ful system will attempt to retrieve that event with the least amount of smear, if you will. Sure, a "bloomier, rounder" system can muddy up an overly-dry recording, but who wants that?
We're talking about time here, NOT frequency response or spectral tilt. The Neuance is one of those "pretty honest brokers" that isolates the component from outside vibrations that smear the presentation temporally,
and possibly excite resonance(s) that excite(s) a spectral coloration. The Neuance thankfully imparts no coloration of its own, as far as I can tell. It snaps everything into focus, including upstream detritus. If such focusing detracts from musical enjoyment because your CDP has digititis, or your amp chain is too lean (can that be?), or your metal domes are just too searing, then don't blame the messenger. An isolation/support device to my thinking should NOT impart a tonal coloration or tilt, nor should it in any way alter (distort) the signal's temporal coherence.
OK, I'll relax now. Cheers.

I agree Ernie that PRAT is an absolute to be striven for. However, some components have tried to be more PRAT-like by actually smearing upper-mids and/or lower treble, creating an emphasis on the attack of percussion instruments, creating a pacey sound but also a relentlessness (I am referring to some of the poorer stuff of some years ago from Naim and others), that may have given PRAT a bad name or created misconceptions as to what it is.

Ernie quite rightly refers to it as temporal accuracy. Those of us that listen for PRAT are trying to find the gear that lets us groove to the timing cues in the music not just the sounds. From much wasted time playing around with supports I discovered (what should have been an obvious) conclusion that a light and rigid support would release its energy faster and therefore do the least damage to temporal accuracy. What is unusual (but there are others that produce similar shelves) about Neuance is that in addition to being very light and sufficiently rigid (provided you buy the right grade for the weight of your component), it is quite damped without the damping causing delayed energy release into the component. I suspect the trick is in how the damping is achieved.

Two other products I used some years ago were quite successful in this regard too. I think both came from Russ Andrews - they were the Torlyte shelves (a wooden honeycomb) that did wonders for my Linn LP12, and Aerolam (aluminium honeycomb) which I never tried as a shelf but did have speakers that used Aerolam for the cabinet.

Just came across this thread. I have just acquired slabs of high quality marble and granite flooring for the new shelving unit I’m having built. 9/16th’s inch thick and 24x24 inches. High traffic commercial quality for class 1 buildings. My experimentation shows both are immune from vibration when placed on a full size piece of isolation material such as  Sorbothane. Zero vibration my new stand will make use of pieces to absolutely prevent the least vibration. No ringing or movement whatsoever.