There are many ways that feedback like this can manifest itself.
It sounds like you are getting low frequency rumble, which is the most common type. But I've also had mechanical resonance occur.
I had two pair of floorstanders, each with twin 8" woofers. I have one pair in the front of the room and the other at the back or the room, and wired them so they were mechanically in-phase. Usually about 2/3rds of the way through an album the tone arm would start jumping around, literally jumping around. I have hit some mechanical resonance of the turntable cabinet and suspension.
You might want to watch your tone arm closely when the 'rumble' is occurring to see if there is any unusual movement.
As you already seem to know, you need some kind of isolation between the speakers and the turntable. I'm suspecting in your case, the problem is the speaker vibrating the stand and the tone arm is picking up that vibration. That is, I think it is mechanical transfer rather than the air in the room vibrating. Especially since the volume seems pretty low.
You can get Deflex polipods pretty cheap, not free but reasonably cheap.
Deflex Polyurethane
The come in small, medium, and large. The
Foculpods are the medium size and can handle 5kg each; 4 for £9.95. These would be good for your speakers. The smaller
Polipods are 8 for £9.95 and can handle 1.25kg each. These would be good for your turntable. You can also use 5 or 6 of these under your speakers, if they are light enough.
Deflex Polyurethane
Deflex Polyurethane
There are several brands of the 'polypods' on the market, but the Deflex seem popular and they are reasonably cheap.
Alternatives, build a small shelf to lift the turntable up off the amp. If nothing else, this is better cooling for the amp. Then set a foam rubber square, .75" to 1" (available at fabric stores), between the shelf and the turntable, that should help isolate the turntable from mechanical vibration. If you don't like the look of the foam, put a pillow case on it, or wrap it in some cloth.
All the response you are getting, are telling you the same thing. You have to isolate the vibration. You have to stop the speaker from sending vibration down into the equipment stand where it can be picked up by the turntable. You will likely have your best results if you isolate both the speakers and the turntable from each other.
This stops mechanical transfer of vibrations. But there can still be feedback from the vibrations in the air. That, though, usually takes some pretty high volume levels.
Depending on how much your speaker weigh, as I said before, you might be able to use 5 or 6 of the smaller polipod under the speaker. Five of them will hold 13.75 pounds, six will support 16.5 pounds. Two sets of the polipods (16x total) would give you four for the turntable, and leave you 12 for the speakers. Total cost about £20. Otherwise, you need two set of four Foculpods at £10 per set, and the smaller Polipods are available individually at about £1.25 each. For a total of about £25.
No solution is going to be free. So, it is down to how much money you want to dedicate to the solution.
Steve/bluewizard