I would start by connecting each amp full range to the speakers. It could be the room but temporarilly eliminating the bi-amp config is where I would start. I've used 6l6gc tubes and they never seemed bright, they are well balanced so I' d be suprised if that's the problem.
How does it sound with other sources? Do you have a BluRay player you can use to troubleshoot?
I have a tough room, 10' high tin ceiling, 6' windows, hardwood floor.... an area rug, and curtains went a long way to tame things. A Velodyne sms 1subwoofer eq/crossover really was the trick to eliminate the boom around 40hz.
Good luck