Thanks, all.
I ended up buying a set of spikes on eBay. The threading was compatible (checked the Axiom one-packs) to be sure.
Upon installation, which took all of three minutes, the spikes satisfyingly punctured the carpet and the pad and achieved contact with the poured concrete floor.
I haven't noticed any sonic changes (no surprise) but I have noticed that the speakers are quite a bit more stable. Thanks for the help, guys.
I'll snap some pics tonight and upload them tomorrow. What's everybody using these days? Photobucket used to be the old standby. I might need a refresher course...
