Subwoofer rumble issue

for better of for worse I use EQ on my power amp (crown xti) to make a high pass filter that slopes steep below 30Hz, this removes any subsonics from record warps. If I place the needle down on the record you see the cone bounce without the filter, you see almost no movement with the filter engaged, I find it very reassuring especially as it should prevent any feedback. It may be a band aid to an underlying issue but It does work.

@paulgardner:
I had the rumble problem for quite a while. I built a suspended TT platform that is isolated from the floor and walls, using dampened springs. I also recently purchased a DBX Venue Driverack 360. I haven’t had a problem since. It incorporates a graphic, as well as a parametric EQ, with “Q” adjustments that come in very handy. The “360” has many other features that I enjoy. You can control the unit with a smartphone or laptop.  ALL speaker’s phase can be adjusted from your chair, as well as ALL speaker’s timing. Of course each speaker has it’s own level control and shelf roll offs. I can even use it’s subharmonic synthesizer for deep bass, although sparingly. If you’re not familiar with this unit, it would be wise to check it out.
(I’m using two JL Fathom 113’s as subs, with my TT platform on the left side wall). Placing your TT in between or in back of your speakers will exasperate the problem even more.

I had the EXACT same problem.  Get some long cables, move the table, it'll fix it immediately.  My subs were on isolation platforms, turntable on a wall shelf with a symposium ultra under it.  I even borrowed other tables, nothing.  The turntable was in a room node and no matter what I did, nothing worked.  I moved it, never had another issue.

I’ve solved that feedback problem on many turntables by dampening the plinth. 

Try a different phono preamp.  I had the exact same problem and spent two years and a lot of money trying to solve it.  I relayed my story to a well known vinyl writer and he suggested I swap out phono preamps.  I was surprised when a friend brought a spare unit over and we hooked it up and the rumble/feedback immediately went away.  My preamp was later bench tested and measured perfectly, it just didn’t play well with my setup and stereo subs.  Good luck!