Really, I just don't see whats wrong with having only 126dB at 12Hz with sine waves. His room is small, he is getting plenty of gain, he is getting lots of output, if he wants to go deeper he is going to have to completely rework his design, I don't see better amps really getting him there.
10000watt, if you are lacking so much things are going to have to change, you are going to either have to find a way to build an IB or several large ported subs, sealed will just cost a lot of money. You could port your 8 subs that you have right now. According to the models, if each EP2500 is just putting out 1000watts, you are getting 111dB at 10Hz without room gain, if you put each driver in a 15 cubic foot box with a 6" port tuned to 11Hz, you would have 124dB at 10Hz with the same power input. Its up to you, but I don't see how just getting better amps will cut it for what you seem to want. If the subs were ported, even with just 250watts from each amp, you'd be gaining 8dB over your current setup if powered with 1000watts per amp.