I think your tube complement is going to be very similar to the Siegfrieds which I owned for several years. The first decision you need to make is to decide whether you want to run in pentode mode or triode mode. This may depend on the efficiency of your speakers and loudness requirements. This may also govern tube preference. 6550s are a bigger brute force tube whereas the KT88 are more musical and refined. The GE 6550s are excellent as mentioned. However I preferred the KT88's by far but I also ran my amps in triode mode so it was a good synergy. As far as brands, I used the Gold Lions and they are very good. But I subsequently changed and bought several sets of KT-88s from Kevin Hayes at VAC who sourced them from Shuguang. They make a particular tube model , KT88-98 that is readily available and affordable from many vendors which I then used as my regular tube. (Kevin charged a small premium for his quality control and "VAC" labeling, but a good vendor can do the same matching and QC on standard KT88-98's which was Kevin's preferred tube at the time). I switched from the Gold Lions to the Shuguangs not so much for SQ (both are excellent) but longevity and reliability (Shuggies were a bit better). You should expect 2000 hours of life for a good KT88; 2500 if you're fortunate. As with all tubes, there is no "best" as personal preference is, well, personal. But the Shuguangs and Gold Lions are both sonically excellent. I have not tried the newer "premium" versions of the Shuguangs (i.e. Treasure series) so there's that to consider as well. No idea if these have improved SQ or lifespan or both. But if Kevin Hayes at VAC has gone that route, that would be good to know. You should also ask Bea Lam, who has some of the best ears in the business. Her opinion would be worth learning.