Turntable feedback rumble problems with new sub

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.