Turntable feedback rumble problems with new sub

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.