If the feedback is ground-borne rather than acoustic, a set of Townshend Audio Seismic Pods under the granite.
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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.
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?"
Best recommendation I can give in your situation is get a wall mount for your table. And get it out of the corner.
I have tried cones and they do nothing to help, one member recommended Iso acoustic footers and I'll second that, it's what I use to very good effect. Just make sure you get the right sized ones for the weight of your table-slab-system that will be supported by them. Also over damping can cause you to lose detail and sound gets sort of muddy so be careful.
I also use a large granite slab 3" thick and 165lb and without isolation it will transmit small vibrations regardless of what other think (sorry geoffkait but my experience dictates otherwise, not to take anything away from your huge experience) its "ringing" in the resonant frequency of the particular piece you have. That's dictated by the dimensions.
Maybe ringing is not the correct term but there is vibration transfer in granite that could be described as ringing that's why you need some isolation, IMO of course. example my case; with out isolation if I tap the slab I can here it clearly in the speakers, no music playing tone arm on the rest. I say its ringing because the tap decays over time not a single short sound. the decay rate is quite fast but noticeable. with the Iso Acoustic footers I still get a slight noise but it has no decay and is much attenuated almost undetectable. So with the Iso Acoustic footers I get the benefits of the mass and isolation as well.
I believe from my experience Isolation and the granite mass need to be done together for best performance, sand box is great idea too if you have the space. who doesn't like to play in a sand box, right?
Another consideration is how much mass do you want swinging away at the top of your stand. Depending on how high your stand is it's a factor that I have encountered that needs to be addressed, think pendulum here.
Remember your Rega is designed to be a low mass table so adding mass will not help it do what it was engineered to do, correctly. you may find it sound best right on the wood top. Or better get-make a wall mount.
I bought a slab of marble in the 80's, and yes, it rang like a bell. Granite, no, it's pretty well self-damped. I'm about to try two granite shelves put together with constrained-layer damping between them. Finding a place that will cut granite for you is a problem, but a neighbor has a wet saw, he's gonna do it for me. Granite's cheap!
if you want stone that has a natural damping feature get slate. costly though.
I've been through all this over the last couple years as I put together my Garrard 301. I was originally going to go with a stone, probably slate, but its quite expensive to get and harder to work with. I finally settled on Bamboo ply layered. I had a slight issues with my granite slab transferring noise to my table when it was on pin points. I tried hard, I tired soft isolation schemes ( many of each type) I eventually forked over the cash for the Ios Acoustic footers and problem solved. I even took the granite slab out but the loss in detail was not worth it so I found another solution in the Iso Acoustic Orea. my tables 100lb'ish sitting on that dam 165lb slab lol I don't have rumble issues lol. bad back defiantly.
Schmitty1 it sounds like the cartridge you are using may be too stiff for your tone arm. You might try putting a more compliant cartridge in there and see if your problem goes away. I have seen instances where bass on the recording was not enough to set off feedback but get enough bass going in the room and away you go. You might also have inadvertently located your turntable in a node so moving it just a few feet might make it less prone but still given enough volume it will feed back. The primary problem is the resonance frequency of the tonearm cartridge combination which is being excited. It should be down between 8 and 12 Hz. A good test record will give you that information.You obviously have a very sturdy platform. I doubt it is the problem.