material for plinth for subwoofer IRS-V

I have a gracefully aged IRS-V set. The fiberglass/resin shell and epoxy-over plywood plinth is still in good shape, but somewhat outdated by what we now know about decoupling and spikes etc. So I am setting out to replace the plinths under the subwoofer towers with either 1 inch or thicker aluminum (24x24) or 4 inch thick solid maple board or even 1/2 inch thick steel. All of the above options will be matched with brass or steel pointed feet. Given the size (8ft) and weight (600lb) of these towers, and the very limited range it covers (sub 70hz only) the conventional wisdom on material (i.e. maple rules) does not seem to apply. Any suggestion?
I would suggest that you get some marble or granite cut (just an inch or two wider than the bottom of the tower), 2" thick and then cover those with a 1" thick piece of laminate-maple. Use a thin layer of silicone sealant between the two layers to damp/mute eachother. Use the spikes then aswell.

Best of luck,
Dewald Visser
You could have custom sized plinths from Symposium. These have been used a brand of Canadien speakers whose name I can't remember.