massive brass plate for isolation

If your looking to isolate a TT especially with spiked feet, I've been experimenting with 1" thick plate of aluminuim braced on the bottom with cross bars of 3/4" square aluminuim. I've had some great results so far as a top to DIY sandboxes. A couple of manufactors will cut plate aluminuim to size for you. A 20x24" plate 1" thick cost me about $200 delivered to my door.

Sorry to not answer the thread question. I really do not know about the brass.

Savant Audio advised me to try using a 2-3" thick Delrin sheet screwed directly up against the base of component and then to use the Walker cones and disks below that.

My sense is that a brass plate would simply transmit vibrations, and perhaps ring into the bargain, as others have suggested. You need Isolation to improve the performance, a slab of any resonant material, absent anything else, will not give you that.