Turntable Resonance Fix

I've been investigating different materials of turntable mats in hopes that I can find one that will help tame a -30dB rumble centered around the 20 Hz region that I fear is affecting the rest of the audio spectrum to some degree. I'm guessing that either platter bearing noise is to blame, or possibly vibrations from the motor creeping into the tonearm. Ultimately I would like to take steps to eliminate the source of the rumble, but I fear that will involve several thousands of dollars (a different turntable). With the vast amount of experience and knowledge out there, can any of you share an example where a particular mat helped with this sort of issue? Baring that, do any of you have advice for altering or upgrading the Pro-ject turntable to eliminate this problem? 

For reference, the setup is as follows:

- Pro-ject 2Xperience turntable (stock)
- Soundsmith Aida cartridge
- PS Audio NuWave phono preamp (digital measurements were taken from the USB output of this unit)

The well-isolated and -damped audio rack sits 25 feet from the speakers. But regardless of that, measurements were taken with the speaker output at a whisper. 

I have taken screen shots of the analyzer that I will post to my profile page.