Subwoofer rumble issue

George- I’ll give that a try when I get home tomorrow. Thank you

  Paul....could also be a bad bearing in the turntable.  If you have a 45 speed, listen for the change in the rumble.  What is your turntable brand

Its unlikely you're getting feedback, unless you have some really lightweight rig with a dust cover sitting on a flimsy particle board rack, something that extreme. In which case, remove the dust cover.

People confuse feedback with rumble and its easy to do since it all seems the same but they're really quite different and its a really hard problem to solve if you can't tell the difference.

There's mechanical vibrations coming from the sub sitting on the floor. There's also mechanical vibrations coming from the rack being made to move by low bass sound pressure. Either way results in added platter/cartridge vibration that can over drive your subs.

A filter is one answer. But this does nothing to address the real problem. The extra vibration is still there, you've only gotten rid of the obvious part to the problem. The guy telling you no one can hear the difference is missing the point. The comparison is not between the filter and no filter, the comparison is between the filter and a rack that lowers subsonic noise down to the same level. Which of course will be a lot better.

The solution is trial and error. It can be as simple as taking something like some lids from a large open mouth jar, turning them upside down, packing with sand and putting the table on them. To at the other extreme filling a box a couple inches deep with sand (always mix with oil so it packs down and stays dust-free) with a platform of wood, MDF or stone laying on top of the sand. To the creme de la creme, a concrete/ABS/sand and granite stand with carbon fiber. https://www.theanalogdept.com/c_miller.htm
You will not get any feedback with this, let me tell you! 

paulgardner OP3 posts11-17-2019 2:11pmGeorge- I’ll give that a try when I get home tomorrow. Thank you

Yep air-born feedback.
I bet nearly 100% it’s your problem.

1: Remember system all on including sub (not in standby auto switching mode).
2: Stylus on a record but not spinning,
3: Turn volume up to your normal loud position.
4: One hand on the volume control, with the other hand solidly tap the turntable plinth with end of your finger .

And watch your "finger donk" on the TT plinth turn into an ever increasing earthquake with airborn-feedback and continue to get louder and louder as it starts to run away and if not stopped will take out your amp and speakers.

Cheers George

The sub is driving the room. Some room are incredibly microphonic. Isolate the sub from the rest of the system by putting it on short stiff springs. You know, like Super Stiff Springs. Trademark. Problem solved! 🤗