Turntable feedback rumble problems with new sub

No! You don't want a bandaid fix.

In the OP's case, the problem is not caused by an electrical issue. so why try to correct it electrically? It's best to fix problems by addressing the problem in a logical way instead of adding unnecessary electrical entities. 

HI,
Try to get a decent rack, adding platforms, granites, marbles and all kinds of that stuff will not have the desired outcome sonically,  for they store some energy, and still the current support will be in question and most propably you will upset orher parts of sonic spectrum. You can then experiment with any kind of footing devices if you want.

Yes, as slaw and others have said, it’s a turntable problem. One that needs addressing asap before you wreck the voice coil/spider assembly through heat.

Some amps have built-in filters to cut subsonic signals from reaching the speakers but most don’t.

Don’t ignore it. My friend had a similar issue with his Pro-Ject Carbon which resulted in a burnt out sub on of his Kef's.

Check everything turntable related. From cartridge tip, to arm, to turntable placement.

As a reward you’ll get better sound too.