Subwoofer rumble issue

what I notice on this topic is that when listening to recordings posted on youtube if the track is from vinyl you can often tell as there is obvious woofer pumping, I assumed wrongly that a signel might have been high passed to get rid of some very low frequency hash, apparently not. Many dance/electronic 12 inches not available on digital format have been uploaded and show this.

I’m having the same problem. I have a Technics 1200 M3D and KEF LS50s. No issues. I just added a REL T7i and have the runaway feedback rumble at high volume. I added Isonoe footers to the table which didn’t do much. Unfortunately the table and speakers sit on a big media cabinet from Design Within Reach which is not ideal.  Not sure what the right next move is.  More table isolation, put footers under the cabinet legs, new turntable, new furniture, or new house? 

I had this when I added 2 Rhythmik 12” subs to my system. Ended up trying 4 iso pucks https://isoacoustics.com/iso-puck-series/ under the subs which solved the issue. 

I'm having the same problem.  A simple experiment demonstrates that it is not "air feedback" - but vibrations through the floor/room.  I simply put the needle on the record and increase the gain until I start getting the feedback.  If I lift the subwoofer, the feedback stops as soon as it is decoupled from the floor.   Any other recommendations for subwoofer isolation tweaks?  I will look at the iso-pucks.  Note that the second subwoofer (across the room) does not create any feedback and I assume that is due to positioning in the room.  I have an old house (230+) and the floors are original.