To begin with I used the Concerto with my Beyer T1's and I loved the synergy there. I have kept the Concerto because I knew that I would either get a T1 or HD800 again. At the moment I just think it sounds great. I am sure the Concerto has the potential to drive the HD800 to its full potential. I think it is only a matter of liking the signature or not. It is very neutral with the Concerto. I hear a lot of details and that amazing soundstage. So far I am not planning to find another amplifier but in the long run you never know.
I have earlier had a few tube-amps and I have a feeling that at some point tubes may be something I should try. I have preferred analytical and bright sounding headphones/amplifiers but with the HD800 I might try a good tube amp in the future.