Nottingham resonance control

You want constrained layer damping to minimize the contribution of your motor to the signal. One layer tries to move against the next, but cannot - it is constrained. The result is heat, not motion. Plywood or slate does this. Brazilian Rosewood does not (although it does look wonderful - a thought - veneer some onto Panzerholz). But prove the concept first.

I doubt very much if metal cones will ring, especially in response to an outside signal. They will, however, limit transmission of vibration. You need to specify shape (cone) not cost. But, they may not work with your system as it is now - I had to insert a cork layer to dampen the sound until I upgraded the tonearm, at which point the cork layers came out and the sound blossomed. Stay empirical, but begin with the physics.

I see. Brazilian Rosewood won't work. Great wood for making guitars, that's why it came to mind.
Not so simple with cones - they remove the vibration at best, then they return some of it. Yeah, shape and material matter. Audiopoints were good cones, no longer made. They work well with my integrated amp.
Probably Vibraplane would be best, but you never really know until you try. Still, you can't try eveything so to a degree you have to guess.
Physics, of course. But at times the laws of physics are so inconvenient.

For everyone's information Nottingham has a new US Distributor. Check out the website audioencountersolutions.com

Boston Audio Mat-1 works great. And also Neuance shelf under the table is a big improvement.

Shakey

Inna, on a follow up note, I removed the little rubber feet from motor and table and replaced them with extra thick Herbies Grungebusters. I have only listened to a couple records so far but I can say with no hesitation they have cleaned up the sound, everything is more concise by a few degrees which in my system is appreciated. Similar experience to when I rolled in some nos tele's. Lyrics that I couldn't discern before can be clearly heard. Bass is easily followed when before it was overshadowed by other players when music got busy. The palpably of the resonance of the stand up bass has increased. A negative brought on was a hardness in higher frequencies that I alleviated by lowering the vta which I had previously raised a tick or two beyond neutral to hear what I could, which I thought was slight increase in detail. Now that I lowered the vta back to neutral it's sweeter with no loss in detail or above mentioned gains. Cost was around 30 bucks and so far has done far more for the sound then I dared hope. Hope this helped in some way. Btw my table is the space294 w/wave mech. and Grado ref. 1