Marble or Granite shelfs in a hifi rack?

Mapleshade likes thick maple, although I suspect it has a signature too. I'd just use whatever's easiest and most attractive for you, and then plop Neuance iso/absorb shelves on 'em for perfect isolation. Birch plywood's usually nicely made, and won't sag, unlike MDF. Don't make the mistake of believing that high mass is necessarily better. Marble certainly has a sonic signature, for example.
Make it light and attractive (NOT glass!), then float the Neuances, Rollerblocks or other high quality neutral devices. Have fun.

I have to agree with Red on this. I find it interesting that the Spectral user finds the Neuance shelves to accentuate the high frequencies. I suspect that what's happening is that the absence of smear and lower-frequency resonances results in a "tighter" (better PRaT, more coherent) presentation that shows the leanness many of us believe the Spectral stuff is voiced to have. I would suggest that room placement and choice of speakers be used to balance a Spectral system, rather than using "muddying" supports.
It IS true that introduction of Neuance shelves under my CDP (EMC-1 MkII) and pre (Aleph P) helped to heighten PRaT, and that quickness and light-footedness can sound leaner as other components' grainniness or edginess is brought into focus. The Neuance allowed me to hear the difference between cables, ICs, and PCs more clearly, and is now allowing me to use a neutral and very fast cable (SPM), for example, as ALL these links become clearer, faster, and less phasey. Neuance really did allow me to "neutralize" any effect of an underlying base material, when set on upturned spikes. I find this aspect more appealing both scientifically and musically than trying to "marry" one of a myriad of base materials to my system's perceived flaws, imbalances, or "character". The Neuance has allowed a predictable and satisfying evolution of my system without a misstep. If I want to change spectral tilt I'll play with speaker/room issues, NOT support hardware. I know I sound like a devotee here, but I love elegant, successful, cost-effective technical solutions to complex problems! Cheers.

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.