Best way to "foot" a subwoofer ??

Concrete floor.It is very difficult to get that bass right on that concrete floor.Most of that room whomping bass starts with a wood floor on floor joists.Are the walls concrete ? Good luck my friend.

Have you thought about a riser ? You could build one out of 2x4's and use OSB(oriented strand board) tongue and groove.Use a ton of liquid nail and screw the hell out of it with drywall screws.Wa-la,floating floor again.Thats how the big boys do it.Get some 2'x4'(4-5 inches thick should do)sheets of owens corning 703 board and cover it with some real porus fabric and kill those first reflections.Build a bunch of RPG skylines to diffuse the rest of the flat surface and you are on your way.Read,read,read.That room can work if you work on that room.