Support table or shelf for turntable

I double what geoffkait just said.

It depends on your approach and your budget.

If you are dead set on keeping what you have, here goes...

Invest in a slightly larger Maple platform of 3" thick and slightly wider than your current rack dimensions. Install (3) Brass feet (into) this platform. Then add another maple platform on top of that decoupled by load rated springs, (hopefully you have the ability to or will have someone line these platforms up and recess the springs by 1’8" inch into the bottom/top.

This will add 5"? to your current set-up? I assume this won’t matter since your components are already in the "line of fire", so to speak.

This will completely decouple your rack from the floor.

Now that your rack/system is decoupled from the floor...you can begin to address your components as geoffkait mentioned.

To mention your marble again.. most materials are not perfectly level. While it may seem the oak platform or the marble mates well, most likely there is a big void in how these items mate, (in audio isolation terms) that compounds the very problem you are trying to avoid. So, most of the time a ball bearing type of interface or a spike is the best way to transfer vibrations. Not always though.

The basics first, is the best approach.

I can appreciate your current set-up.

Upon reconsideration, I believe you could now see improvements from a total isolation/sonic perspective? Right?

Good luck!

You may find this helpful in explaining the benefits and pitfalls of different solutions: http://symposiumusa.com/UltraAnalog.html

I like the idea of both "isolating" the rig from external vibration while simultaneously draining the plinth and bearing of vibration through transmission to the constrained layer platform.

I am in the process of installing an Ultra Platform under my TNT with a separate custom Super Plus mini-platform underneath the motor/flywheel assembly, all on top of my existing BrightStar Big Rock/Sound Anchors stand. Hoping to complete the project this week.

Dave

Thanks everyone for the valued suggestions. I’ll study up in more detail all the notes kindly supplied. Thanks.
A few points to clarify:

1. Remember the new rig will (hopefully) be a rebuilt 401 in a slate plinth.
   
2. The issue I’m facing is that when I knock on the oak table legs or lower cross-member, I hear it through the speakers. I reckon if I can hear it, so to can the 401.
   
3. I want to get rid of the oak table anyway and take the LP12 out of the equation. It was a temp solution years ago and now i want to deal with it.
   
4. The granite blocks: I originally had the TT on the concrete floor on felt pads. It sounded great. I moved it to the oak table and it lost a lot - woolly and smeared. I added the 3 x granite blocks and it made the world of difference - 90% of the floor set up. I then added the steel cones pointing up and touching the underside of the plinth, and this made it sound better than the floor. This is the way it has been for a few years. Because of this experience, I feel the 401 works best when everything is as rigid as possible.
   
5. Before going the (complex) springs route, I’d really like to try something like a wall mounted table. See these brackets. Wall mounted with upward pointing spikes into say a 2" maple top plate. I’m uncertain what type of feet to add between the slate plinth and the maple. Rollerblocks?
   

I use cones  (Bearpaws) under my turntable and found it a very valuable upgrade.  Tried other things without that level of success.