Not to be negative, on your subs (as they are great subs), but I had the same issue in my basement room (concrete floors), i tried them all, 10", dual 10", single and dual 12" and a single 15", and in the end I choose to build some custom 15" boxes tuned to 18 hrz and used the KA-1000 subwoofer amp to power them. Even then it took some time to find the right placement for them, but in the end, I ended up with the sound and feel that I was looking for. I know that not everyone is willing to spend 1k on a subwoofer amp, when you can buy a really nice powered sub for that price, but for less than 2k I have dual 15" subs that can hit 18 hrz all day long.
I love your room, nice and clean great colors, and your pictures are awsome!! good luck with your subs.....
~ ross