They sound great to me. I am currently running Tung Sol EL34B's in my 3 channel Triple, and it took away some of the sub-bass frequencies, and gave the amp a little more of a mid punch. The sound seems a little more "focused", I guess because of the "tighter" bass response. The other guitarist in my band uses amps that run EL34's specifically, and the Mesa with EL's really fits in with their tone. The thing to do is to basically start over with your EQ settings, because my 6L6 settings were not a good fit for EL-34 tone. I might go back to 6L6's one day, but, for right now the EL-34's are doing the trick for my ears.
-AJH