Dear grimace: It happen that my TTs has ( add-on ) a vaccum system too and I use a clamp that like in your case makes a quality improvement.
Put a record with the vacumm working ( with out clamp ) and hit with your finger the LP near the center ( you can do it any where but near the center is more easy to hear. ) and then make the same but with the clamp-on and hear that this time the " sound " from the finger hit on the record is more " dead " that with out clamp, resonance/vibration comes lower and this means lower/diffrent distortions in favor of the music sound quality reproduction.
regards and enjoy the music.
Raul.
Put a record with the vacumm working ( with out clamp ) and hit with your finger the LP near the center ( you can do it any where but near the center is more easy to hear. ) and then make the same but with the clamp-on and hear that this time the " sound " from the finger hit on the record is more " dead " that with out clamp, resonance/vibration comes lower and this means lower/diffrent distortions in favor of the music sound quality reproduction.
regards and enjoy the music.
Raul.