I haven't tried any 6550's yet. I do have a full set of EH EL-34's that came with the amp when I bought it. The guy I bought it from said Mark O'Brian from Rogue likes the EL 34's. He liked the KT-88's and I haven't had time to play around with it yet so I have left the KT-88's in. Have you tried any EL-34's?
I did get some NOS Mullards from Upscale Audio for input and driver tubes. What a difference! They really smoothed them out. According to Rogue the input tubes will make the biggest difference. I'm really pleased with the amps and am anxious to upgrade the pre.