Come on now Marty let's quit picking on poor litle ole' Martha...
John the VTA is analog-speak for Vertical Tracking Angle. It's the angle of attack between stylus & record groove surface. By addng a mat, effectively raising platter height, the VTA changes which changes the sound. Many better tone arms have adjustable VTA for optimizing sonics; presumably yours may have this feature with your VPI/JMW arm (unless perhaps you have a Rega arm, which isn't adjustable in factory form). Lots of information is available on properly setting VTA; these posts can also be found here by using the forum archive search engine.
So what I'm saying is that all mats have different sonic signatures by themselves, but by adding mat height which changes the VTA, there is an audible effect there also. In order to hear the effect of the added mat alone, you'd have to measure your present VTA (& assuming it's already been optimized by whomever setup your arm) then adjust the arm height for that same VTA. Make note of where you started out (count the turns on the dial etc) so you can always get it back to where you started.
For the uninitiated, here's a link to George Merrill's outstanding & inexpensive turntable setup booklet:How To Setup A Turntable
There you can also learn how to optimally adjust your VTA.