Subwoofer rumble issue

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! 

First look to see how much subsonic turntable rumble is getting through. If the woofers flutter a lot at high volume even with no signal, like on the lead-in and lead-out grooves, or between tracks, that’s one thing.

Also does this ever happen playing CDs?

If you have no problem playing CDs, and especially if you also have a lot of flutter, then yes you need a better platform.

Just not any sort of isolation platform. There is by the way no such thing. What there is is vibration control. This false notion of isolation makes people forget what is so plainly evident, that a box full of sand ain’t gonna isolate nuttin but it certainly will control and eliminate a lot of troublesome vibrations reaching your turntable.