T-6061 aluminum air hardens. By this time that amp stand must be like a trampoline-very lively. Not exactly what you may be looking for.
Granite (Harbor Freight has the machine plates on sale from time to time-free shipping also) is harder and more lively than marble. Granite is also a bear to work with without diamond tools.
18x18 inch marble tiles are usually available from a tile/carpet store. If you buy close-outs and partial boxes you can get it real cheap. 18x18 x 3/8 thick #3.00 USD per tile and free cutting to size if you ask nice (on a store wet saw a piece of cake). Again, marble has a low shear strength so it needs full support underneath at least on the amp contact points.