In my case, with a Classic 1, VPI recommends against a mat. To reiterate the original question, what's the theory?I explained the job of the mat- it is ideally a damping device.
The problem is that when the stylus tracks a groove, the vinyl resonates a bit and so can talk back to the stylus. By damping the vinyl, the talk back is reduced and so the resulting signal has less distortion. You can tell this is happening; if the LP is being damped the stylus will be inaudible with the volume all the way down. If the LP is not damped, you will be able to hear the stylus tracing the groove from across the room.
I´m not excited in damping the vinyl, damping the platter (from motor noise) is completely another thing.You can't damp motor noise with the platter pad. If the motor is not sufficiently decoupled (IOW its noise is apparent when the stylus is in the groove) then the turntable needs repair or replacement. It really is that simple. I agree that acrylic mats don't sound right- they are too hard and don't match the durometer of the vinyl so energy is reflected back into the LP and picked up by the stylus.