Marble or Granite shelfs in a hifi rack?

Subaruguru, I suspect that what you are saying and what I am saying are in agreement. I am familiar with the prat, lightness, and also leaning out effect of Neuance. I believe, without supporting measurements to say so, that Spectral/MIT is ALREADY more time and phase coherent than most other designs. Neuance helps to move other equipment toward the same coherency and resolution by eliminating smear of the signal. The leaness of the signal that you speak of with Neuance, and that you also attribute to Spectral, may be an inherent result of tightened time/phase coherency.
This is not our understanding of live music. Fullness, bloom, continuity are normal in live music but we are discussing recorded cd's, wherein an entire chain of recording and playback electronics have acted on the signal. It is possible that tight coherency in all aspects of playback equipment can be carried too far, to the point that low level distortions, filter artifacts, a/c line noise, transformer artifacts, and digititis in cd's may be resolved and amplified.
Also, I mention Spectral/MIT as an entity. Neuance has the greatest effect on power supplies, preamp, and a/c filters, The first group of components may be more subject to transformer artifacts than the latter.
Changing speaker placement, speaker choice, or room treatment won't eliminate a need in the end to deal with vibration and a/c. I'm speaking about small refinements to a system whose character I already enjoy and prefer.

Just to get back to the original question for a moment - and not discounting my belief in the Neuance approach - my experience with Marble versus Granite depended a lot on the Marble being used. Fortunately, for once, the cheapest Marble sounded the best. The cheapest Marble has the most impurities in it and the glass-like ringing of the Marble is much reduced.

Compared with the ringing of Marble, I found the ringing of Granite to be more of a 'boing' than a 'ting', if you know what I mean, a heavier slower sound than the Marble, but this varies by thickness. The use of soft footers between the shelf and the Marble/Granite, as suggested by Albert is the best configuration I could find with these materials. The EAR feet are good, but a friend tells me he has swapped the EAR for another compound that he says damps the ringing more effectively. I am taking a couple of Neuance shelves around to his place shortly and so if the compound is readily available I will report back.

In the end I preferred the Marble over the Granite.

I have read Ernie's post more thoroughly now and believe he is saying something quite important.

Introducing any new component to an existing system is a big ask since if it replaces something that is say bright, then it will sound relatively dull. Does that mean the new component is dull? Not necessarily.

In my philosophy of how to put a system together, I believe that you want as many components (I include cables, supports etc into this) to be "honest brokers" of sound. This is for two reasons. The first is simply that I believe the sound will be better when most components are "honest brokers" than when we mix flavoured components together to try and get a system to be an "honest broker". The second is that you are in a better position to improve the system component by component if each component is an "honest broker" of sound.

I use the term "honest broker" to mean a wider range of issues than just tonal neutrality, such as phase issues, PRAT and dynamics.

I reckon Ernie is saying something that very much accords with my view that in the Neuance, I have found a shelf that appears to be an "honest broker". Hence it is a uniquely valuable thing to build a system around. I may very well be wrong here. The establishment of whether a single component is an "honest broker" of sound is fraught with problems since an individual component can never be heard on its own.

But if I am right it should come as no surprise that introducing an "honest broker" into an established system may cause it to tell the truth about the rest of the system.

But there is no way that I can prove to you the Neuance is an "honest broker" of sound. Only say that it appears to be one, to me, and I have tried the Neuance in several systems.

I can certainly agree that the shelf itself cannot be considered in isolation. Presumably the best shelf for a plastic cased component will differ from a steel cased component will differ from an aluminium cased component, etc, and a component that rigidly couples the circuit board to the case will differ from a component that rubber mounts the circuit board etc, and a component that has moving parts will differ from one that just has electronic parts, etc.

In my experience the Neuance works well with an appropriate rack in most circumstances, and the main variations occur with turntables, CD Players and valve preamps. I would add however, that your own personal values come into it. I have tried the Neuance in two systems where though I preferred the Neuance, the owner did not. In both cases the owner preferred the bass presentation of using a heavier shelf. In my opinion the Neuance is more correct and musical in the bass - being faster, more rhythmical and with more slam at the front of the note. The downside for the owners of the systems was the apparent loss of weight.

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.