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!
paulgardner
George, Dan - thanks again for your insight and ideas on how I might go about resolving this! I did end up ordering a subsonic filter from KAN Electro Acoustics. Fingers crossed it at least help because relocating the turntable to another room just is not an option given the space/layout I’m currently working with. When the filter gets here and I get a chance to try it I’ll post a quick update. 
Paul
This filter you bought, is for very low arm cartridge 7-12hz resonance (below 20hz), this you can’t feel or hear, it’s just very slow woofer movement.
I think from what you described you problem is far higher, around 30-70hz this you feel and hear like an ever increasing earthquake.

If you have floor standers with half decent low bass as your main speakers, try doing the "donk" test again but with the subwoofer/s turned off this time. try it even if they are just stand mounts also.

Cheers George
And what’s causing the problem with the table? The subwoofer. The REL is driving the room. It’s the structureborne vibration, seismic plus Rel. The resonant frequencies Fr of the cartridge and tonearm are circa 8-12 Hz, the platter can be even higher. Some rooms are incredibly microphonic, especially the upper floors of buildings. Some racks/stands amplify the vibration. The REL weighs only 42 lb so my cryo’d Baby Prometheans would be sufficient to properly isolate the REL. $48 free shipping, cannot beat with stick!
Just out of curiosity, try inverting the phase on the subwoofer and see if it makes a difference.