From My Experience and these following comments are from a personnal view point.
Not using a Turntable Support as an example, as there are major benefits to be discovered from using permutations of Isolation/Support under one.
A Isolation Placed Under a Shelf/Sub Plinth on a Rack has not changed anything to the point it is recognised as a noticeable improvement.
From recollection the use of the Isolation might alter the perception of the way the device is presenting, but this could also be imaginary.
Using the same type of Isolation that has been used under a Shelve/Sub Plinth and used as footer separating a Device from the Shelve/Sub Plinth Material has shown more meaningful and noticeable changes to a Devices performance.
A delivery can be perceived as ranging from Bass is Light Footed to Bass is Heavy Footed, depending on the Footer Material and Design.
I try and keep these types of footers not too complicated as design and a affirdable to source material.
When a Footer is discovered that is seemingly adding a value to a device being used, I will then use this footer under the Shelve/ Sub Plinth as a measure of a self satisfaction, whether needed or not.
I have trialed with different Shelves/Sub Plinth Materials and MDF has not been a preferred material.
I also trial with Tiers of materials to produce a Shelve/Sub Plinth and MDF has not found its way as go to material in these permutations either.
It has been a long time since MDF has been trialed so I can't quite recollect why it was not desired, but any material that I perceive as
Bass Heavy Footed is rejected by myself, as this in my mind is smearing the details and suppressing the Mid's to Highs of the frequency range.
Most of my Shelves/Sub Plinths have a Granite in use as one of the Tiers.
Different Densities of Foam are regularly used as well and a Cork Pad that will usually be found as separater of the the materials from the Supporting Frame and be used under a device as the footer.
After various Permutations of Materials to used to support my Mono Block Amp's.
I have produced a Sub Plinth, that consists of a Material used under Laboritory Floors to absorb Micro Vibrations, this is Multi Tiered and has produced two Piers to a Height of 500mm.
Sitting on the Pier Material is a Granite Lintel that weighs in at 250Kg (550lb)
Seated on the Granite Lintel that Bridges onto the Two Piers is a Layer of Hard Foam which offers a resistance to being compressed, as well as a Layer of a Highly Compressed Foam Board which has structural. properties.
The diffference of the perception of the Amp's presentation when lifted from the Floor on a Sub Plinth to the seating on the Granite is immeasurable, it was a instant WOW!.
The fine tuning with the Foams as the final tiers in the permutation was more of the obsessive personality than essential, but the Foam Board is my go to material as much as Granite in a Sub Plinth that has additional tiers.
My Hifi Racks Rigid Frames and all the Shelves are with a Granite as a minimum sitting on a Cork Pad.
This Rack was a few years ago seated onto a Purpose Produced Sub Plinth Support.
Again this added treatment was perceived to have produced a small improvement, but most importantly it gave a reassurance and future endevours are starting from the best base.
Starting the experimentation is the key, finding a material of permutation of materials that are perceived as a time well spent and a noticeable improvement over the previous used will be the reward.