has anyone with concrete floor had issues with the sub rattling off the floor.
I use a gym "puzzle mat" square (similar to this one) under each of my subs. They're cheap, and they work great.
+1. I have two PC12-NSDs and can confirm all you say. Also, with dual subs it is much easier to get a good, smooth response across the entire room, with far less seat to seat variance. Also helped me with smoothing out room modes - much easier than doing it with one sub.