Turntable feedback rumble problems with new sub

Son in law had a similar problem at loud volumes with Def Tech SM55 monitors sitting on the same cabinet as turntable.  We put Vibrapads under the speakers—helped a lot. Upgraded to IsoAcoustic Isopucks, helped even more. I have Isopucks under the speakers in my second system as well—made a huge improvement. Gaia IIIs under speakers in my main system. Big fan of IsoAcoustic. 

I just tried moving my speakers from the console to the floor. Made a huge difference. I’m going to order stands for the speakers and do some additional isolation under the turntable and under the furniture feet and see if that eliminates it completely.  Stands are a tight fit in the room but necessary even if I tried to change the furniture. Thanks for the input all. 

LOL
Speakers and turntables shouldn't be on the same surface (console). 
Speakers on a proper stand on spikes 
Your turntable already have a very nice Isonoe feet 

I had a similar issue with my turntable and REL 528SE s/w.  The first time it happened, I had bought an Auralex Audio Subdude II platform for the s/w.  The rumble was pretty severe and after trying out many things, realized it was the platform that was causing the rumble. I ended up returning the platform and Blu Tak'd the subwoofer to my wood floor for a more stable connection.  The second time it happened, I had to replace my regular legs on the HRS platform that my turntable was on with special low-frequency legs. The cost was $550 for four legs and it's worked like a charm. This was actually a recommendation by HRS and confirmed by Franc Kuzma as I had just bought the Stabi R turntable. Unfortunately this could be a common problem with analog rigs and a pain to troubleshoot.  

ronribbons,

The LS50s must have been triggered by the REL sub and feeding back into the turntable via the cabinet and then feeding back into the sub etc.

As rdk777 said, this is something to keep an eye on in future.

Glad you’ve sorted it, you should also get much better sound now.