Hey,
I am currently using a quad of EH 6550's. IMO, an excellent tube. They are much better than the Sovtek KT88's I have. I will be buying a quad of EH KT88's from Jim McShane next week.
I would like to try a trio of Mullard's in mine. I have a trio of NOS Jan?Phillips 12AT7's in it now and they do a real good job.
-Vic