Turntable feedback rumble problems with new sub

Unfortunately the table and speakers sit on a big media cabinet from Design Within Reach which is not ideal.

Sounds like the media cabinet isn't doing its job. Can you hear foot falls when you walk past the turntable while its playing?

Other than placing the Table in another room its a hit and miss
scenario.
I assume your not using the table with the dustcover down
while playing or have the sub level and crossover adjusted 
to high?
As noted there are lots of tweaks but what many people 
continue to look the other way from is adequate room treatment
which will ameliorate room modes.

All you need is a couple inches of sand in a box. Pack the sand, put something on top of the sand- can be one big sheet of MDF or four small ones, hardly matters- and put the turntable on that. The footers you have may work better under the sand box, or under the turntable, you will just have to try them to see.  

If you have the space and the funds you can do even better with a granite machinists plate, which is what my table is on. Between the granite and the concrete rack is an inch of sand. On the bottom shelf the phono stage is on sand and if you look close you will see the square MDF footers the phono stage is on. Pic #7. https://systems.audiogon.com/systems/8367
The sand is mixed with a small amount of mineral oil that helps it pack smooth and stay put without dust. Dirt cheap, tried and true. 

@atmasphere

Sounds like the media cabinet isn't doing its job. Can you hear foot falls when you walk past the turntable while its playing?

Not really, but if the needle is on the record and you walk in front of the cabinet you can see the subwoofer flutter, so it’s definitely picking it up. It’s a wood floor. I don’t have anything under the cabinet legs, which are metal. 

If you have Isonoe it should be fine even if there is 10 000 Watt speakers near. Do not screw Isonoe tightly to the Technics cabinet you will kill its spring effect. It must be like this.

Move your SUB somewhere (away from the turntable).

@ronribbons I used to have that problem. I placed a set of bearings beneath the equipment stand, allowing it to be coupled to the floor, but otherwise able to move from side to side a little bit. This solved the footfall thing.
I used Aurios Pro bearings https://www.musicdirect.com/vibration-control/aurios-pro-max-isolation-bearings-set-of-3But they are n/l/a. But you might be able to find something similar.

I like the Idea of using granite, I went to the stone cutters place and when they make an area for a sink they have to cut out a big square piece of granite and then toss it out! I was able to get a nice size piece for the top of my stereo rack, it works very well at dampening all vibrations and resonances.

Maybe put your subwoofers on isolation pads. I think you can buy most Auralex products on Amazon

https://auralex.com/shop/iso-series/

Springs man.

If you'll look back and search discussions on tt isolation, there's plenty of info still valid today.

Upon further review, even with the sub turned off, the the LS50s start to rumble if needle is on record and volume is turned up.  I never really noticed this occurring, probably because I wasn't looking for it.  I've heard that old Technics 1200s have a "rumble" issue, so I'm wondering if its present regardless of isolation efforts.  I ordered some Herbie's Fat Dots to put under the speakers, although I might try them under the Isonoe footers to see if that helps.  @chakster I adjusted the footers as you suggested, which didn't eliminate the problem.  
@millercarbon I don't think I have the skill or tool set to build a sandbox, but amazing pics.
Any thoughts on utility of turntable change, or keep tweaking the isolation.  I haven't seen stuff on springs but I'll take a look.  

Townshend Audio Seismic Pods under the table, the speakers, and the sub.

It’s your TT support brother. You need to decouple your TT from all other negative forces.

Remove the speakers from the turntable cabinet and use speaker stands. Or... remove turntable from cabinet and use turntable shelf.

other end solution, at least partially is a phono stage with a subsonic filter - like on the better lehmann phono stages

No! You don't want a bandaid fix.

In the OP's case, the problem is not caused by an electrical issue. so why try to correct it electrically? It's best to fix problems by addressing the problem in a logical way instead of adding unnecessary electrical entities. 

HI,
Try to get a decent rack, adding platforms, granites, marbles and all kinds of that stuff will not have the desired outcome sonically,  for they store some energy, and still the current support will be in question and most propably you will upset orher parts of sonic spectrum. You can then experiment with any kind of footing devices if you want.

Yes, as slaw and others have said, it’s a turntable problem. One that needs addressing asap before you wreck the voice coil/spider assembly through heat.

Some amps have built-in filters to cut subsonic signals from reaching the speakers but most don’t.

Don’t ignore it. My friend had a similar issue with his Pro-Ject Carbon which resulted in a burnt out sub on of his Kef's.

Check everything turntable related. From cartridge tip, to arm, to turntable placement.

As a reward you’ll get better sound too.

Please report back once you've tried at least 5 things and while
using the sub you still have issues.

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.

As an update.  Got KEF stands for LS50s. Helped. Then Herbie’s Fat dots under the cabinet. Helped more.  Then Herbie’s Grungebuster Dots under the turntable feet. Problems almost completely gone. I have to tap on the turntable to start the feedback, but won’t occur with stylus resting on the record. It appears there is no limit to the isolation steps one can take. I may get a butcher block for under the table, or I almost have my wife convinced we need a different cabinet. Thanks again.