Turntable Rumble/Feedback

I had almost this exact same thing, only my rack is a lot more massive so mine wasn't as severe. But same exact thing. Right down to Nobsound under the turntable. 

Problem is different things isolate at different frequencies, and Nobsound are not effective enough at this frequency. Replaced them with Townshend Pods, and that was the end of that.

You might be able to get away with moving the Nobsound to under a sub and using Pods under the turntable. But as severe a problem as yours might well call for the Full Monty of speakers on Podiums, subs on Pods, and turntable on Pods.

I would try slaw suggestion first, remove some springs it looks like your Nobsound could be compressed more. This will shift the resonant frequency lower, possibly low enough to suffice. So try it. But don't get your hopes up. Mine were optimized, still no luck. Nobsound are great for the money, but just not in the same league as Pods and this is one of those areas that highlights the difference.

Looking at the pic, to me it just looks like a whole lot of vibration going on in a confined space. Is the hardwood floor on concrete or is there a basement, crawl space underneath? I know it would be a pain, a rug or runner underneath your rack and sub could not hurt. Double check your sub crossover to make sure its as low as possible for your speakers. You have some big ol' speakers and subs moving a lot of air in that space. Keep curtains closed. What would happen if you moved your rack to in between the main speakers? 

I can’t get over how close that type of sub is, to the rack and it’s NOT decoupled. It’s on a mat? That won’t work. That type of sub rattles the heck out of the floor plane, the one that is CLOSE to your TT. Bla, Bla, Bla everyone, that won’t work, I don’t care how it sounds, "it sounds best there", that does not mean it sounds good at all. There is ZERO dampening in that set up. Everything is going to vibrate. It’s not hard to SEE if you just look. No carpet, traps, drapes, and then pack a sub right next to the rack..

There I said it.. Sorry if I hurt anyone feeling.. LOL Like I give a crap.. Ground up, not paint first.. It’s like talking about 600 horsepower engine in a car with a flat tire and a broken windshield. Fix the flat and the glass, first.. Mercy.

Happy new year..

Based on recommendations here I ordered modeling clay from Amazon and will pack the open cavity in the plinth with it when it comes. I also moved the sub further away- about eight inches, lol. That’s as much as the servo control cable would let me move it, it’s not as long as I remembered. The sub is on a sheet of MDF and the MDF is on sorbothane feet, I suppose I could add an extra layer of each to try to further decouple. The sub crossover is set in the Velodyne controller at 85HZ and is not adjustable, unfortunately.  As for the Nobosounds, I had tried removing springs on the old rickety rack to no avail, it’s a good idea to repeat with the new rack to evaluate. Also, ordered a rumble filter. I appreciate the help here guys!

There is some talk of filling the underneith of the plinth with modeling clay to dampen the plinth.

Are you sure that you listed the correct model number for your turntable?  I searched on Google and couldn't find it.

If it is an older JVC model, the above "fix" may be very beneficial. 

I have a JVC QL-Y66F.  It's a beautiful table, but the plinth is super resonant.  I stuffed a few pounds of plasticlay inside and replaced the feet with feet from MNPCTECH.  When I first got it, it howled like a banshee.  After taking those two steps and also putting it on a Symposium Ultra platform I could play it as loud as I wanted with no issues other than a little woofer pumping at high volumes.  That was with a Rythmik dual 15" F25 sitting literally right next to it.