Turntable feedback

If the feedback is ground-borne rather than acoustic, a set of Townshend Audio Seismic Pods under the granite.

Yes its possible no matter what the table is on the Planar 3 itself can still vibrate if you get the energy in the air high enough. Especially if you're using it with a dust cover. If you are that would be the first thing to go.

Granite is great for mass and dynamics. My amp and turntable stands both use granite. But granite also rings. Badly. My turntable rack greatly reduces this with a sand bed under the granite and BDR Source Shelf between the granite and the turntable.

Try putting the granite direcly on the table top. Try something like sorbothane or blu-tack between the granite and the table. Anything, absolutely anything, will be better than what you have now. The points aren't isolating, they're actually maximizing vibrations by allowing the granite to ring.

Also highly recommend a small sand bed between the turntable and the granite. Won't eliminate feedback coming directly through the turntable but it will be a big improvement over what you have going on now. 

I would strongly suggest that you locate your turntable away from a corner of the room where standing waves converge.

what cart are you using?  Back when I had a P3 I found that some of the higher output MMs created a lot of feedback.

millercarbon

Granite is great for mass and dynamics ... But granite also rings.

I’ve seen others make this claim and it confounds me every time. I have two granite shelves in my system, and each of them is deader than dead. The only thing that happens if I rap it with my knuckle is that it hurts my knuckle. And if I tap it with a hammer, it’s still deader than dead.

So I have to ask: On what do you base the observation that granite "rings?"