I might try the HW40 feet I have , they seem way shorter than the Prime Sig feet so not sure all will line up with motor?
@tommypenngotti Yep I had to shim it with hockey pucks as needed to line everything up. The HW40 feet will help for sure - it’s worth it rearranging for.
The issue as you describe it is extremely similar to what I’ve fought with. In fact with some of my worst configurations, it could get excited to an almost runaway level of that rumbly / hum noise (grows in level on its own, until I mute). And yeah, acoustic treatments won’t do a damn thing about it, either. Best I can tell, the problem is due wholly to structure borne energy, not acoustic.
The top shelf and its support underneath is what determines how much of this energy your table has to deal with. The VPI arms are sympathizing with the energy (less for the metal wands), and the table plinths don’t do much at all to stop it. Squishy feet like HW40 are really your last line of defense. Think if I had a rack with cross-bar support level (at least for the top shelf), and support discs positioned more toward the interior of top shelf, that would help a LOT (e.g. higher-end Critical Mass, or for a LOT less money Adona Reference racks, etc). As it stands, my Sotto Voce rack only supports its top shelf along the very front and back edges (with a few simple rubber bump feet) which I think is a big part of the problem in my case - the CMS Platinum shelf, substantial as it is, acts like a drum skin at the problem frequencies.
Here’s how I "test" a given configuration: lower the stylus in a still groove (turntable at rest!) and turn the system on / un-mute at your normal listening level. Modestly thump the top shelf near your table with a few fingers. That "drum" like feedback level tells you how bad that config is. The worse ones will be much louder per thump, with a very long-lasting ringing effect. Get a feel for what it is now so when you change configs you can meter it against that. An iPhone can record these frequencies just fine, so take videos for comparisons. However, you will need to playback with decent headphones to reproduce those frequencies.
The problem (obviously) gets exponentially worse at higher SPLs. Guys who cap out at 70 - 85 dB are going to be like like "huh? my table is dead quiet!" because they won’t ever hit that threshold to excite these resonances. I listen loud (up to mid 90 dBs continuous), and yeah in this small room with bouncy floors it’s a struggle. My main rig is in a bigger room with much more solid floors, and it was easier (also that table is on a CMS Maxxum which is much better).
Which exact VPI arms do you have, and which is the one that’s been the bigger problem? The metal arms are just much quieter at these frequencies, in my experience. Sure they might "ring" at high frequencies, but that is MUCH lower in level and easier to deal with.