Turntable feedback rumble problems with new sub

@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.