The mat itself can alter tonal response, but whether that can outweigh the change in VTA is the question. I'd take the mat off and adjust the height of the 309, especially as that is easy to do.
Help with tone arm geometry and bass
After installing a new cartridge, I checked that my SME 309 tonearm was level. The sound of voices especially male voices was a bit bass heavy. so I added a ringmat turntable mat which raised the level at the head shell by 2mm. The bass heaviness disappeared. This is the opposite of what I was led to expect. I always thought head shell up meant tail down and more bass. have I got it wrong?
I would like to dispense with the ring mat so I can use a record clamp on the spindle. But this would mean tone arm level again. Which way do I adjust the tone arm to remove the bass heavyness without the mat?