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.
Subwoofer rumble issue
I’m having a hard time solving a problem that seems to come and go. My subwoofer will rumble uncontrollably when playing records at a higher volume. If I turn the volume down sharply it goes away but when I creep the volume back up it comes back. I’ve tried isolating my turntable as much as possible with diy books, isolation feet, etc. (I don’t want to buy an expensive isolation platform unless that really is the issue.) Any thoughts on what I can do to fix this? Thx!