Bagged gravel or sand sounds the way to go, sonically if not aesthetically. Don't know if I could live with the weird shape though, but it would be lot cheaper than the 20mm thick granite slabs, which were estimated at £120 a pair cut to fit the top of the speakers, still leaving me with a flat surface for album cover etc.
Linn Axis tt/Akito arm/Nagaoka MP200 or ATVM95/ML carts/Trichord Dino mk3 phono stage/dedicated PSU
Marantz CD6006/Cambridge 650T DAB
Tisbury Mini Passive II pre/Roksan K3 Dac/Roksan K2 integrated used as power amp
Atlas Element Integra Superior inter-connects
Spendor SP1s(1988)/Van-Damme UP-LCOFC HiFi 4mm cable