Thanks for the replies guys.
Doug, it's funny you mention the slabs. I just took them off some 5" thick Vermont granite I found as part of a large landfill project. That solved the stability problem, but raised the speakers much higher than they were designed for, and made for a larger visual presence which didn't sit too well with my wife. :-)
Rose, I think I could get away with one set of the Soundocity outriggers for the back, where the Tantos only have one spike (that's the problem right there). That may be my best option. The OEM Gemme outrigger feet do "look" better however.
I hope the Gemme guys are still around. The Tantos are an amazing speaker and I hope to get an opportunity to hear the Katana's at some point in the future.