In the recent past, I have heard the AT 666 Vacuum Mat as a Comparison demonstration to a range of Mats with a Spindle Weight in use.
I will say, I was overly impressed with the AT 666 Vacuum Mat in the time I was experiencing it in use, and will hopefully one day have an opportunity to have an extended loan, for it to be used in my own system.
From recollection the immediate and most noticeable effect the AT 666 has on the presentation was to create a perception of a Lean Bass Note and a increased projection of the Mid and High Frequencies, to perceive this happening is not strange to myself, it happens when I use a Metal Mat and especially a Tenuto Mat and Spindle Weight.
The owner of the AT 666 also has a 5mm Tenuto Mat, so to perceive the AT 666 Bass Notes were quite Lean in comparison was not easy to settle with, as it suggests there is need to match it carefully to a system.
This does not put me off the idea of owning one, as I carry out a fair few Platter Mat demonstrations in other systems and the changing environments the mats are demonstrated in always creates mixed results.
In the most recent demonstration session, the notes for the day refer to a Zavfino 1mm Leather Mat placed on the Tenuto with Zavfino Weight seemed to get the majority of appraisal, but again the notes from the session, also describe the two differing materials for the AT 677 and AT 600 as being able to present with a impression that is quite attractive as well.
When doing comparisons such as above, there is a tendency for the change of presentation to be detected, but not so much the benefits the material is offering overall, time constraints are at work.
It is best to live with the materials for a period of time and see how the benefits overall can be detected, In my case during a extended assessment, I will be listening for the micro details being perceived as clear and not with distortion, especially not being perceived smeared. A note or vocal is assessed for having a improved envelope showing the shaping. When these types of influence are detected, this is where I place the real value for the addition of the material. A material change, and detected change to the sonic only does not cut it for myself.
When a interface between Platter and Stylus, ' is discovered ', that creates a condition that is showing the Cartridge function is the beneficiary, it is very hard to take on board suggestions that could mean the end result, is that the perceived ideal condition is to be veered away from.
My experiences have aligned myself to thinking there is not a ubiquitous answer for all environments, the methods used to mount the TT and the choices made for Mat Materials and Spindle Weight Type, are to needing to be trialed and go through a selection process, to get the benefits that are most preferred and noticeable for being attractive to the end user.