which power tube sounds better: EL34 or 6550

This depends on the strengths and weaknesses of your amp. The El34's are great in the mids, but a little weak in the bass and rolled on top with my RM9 MKII. The 6550 Svetlana's are great , but they lack the lush mid of the EL34. The 6550 has an iron-fisted bass, better than most SS amps I have heard including some Krells and the like. Buy two sets and trade them when you want a change. Jallen

I have a pair of RM9 Mk.2s (bi-amp), with original EL34s, and it's time to get new output tubes, so this is very interesting! I think I'll stick with EL34s, based on your opinion, and others I've read.

@eelgar1857 The RAM EL-34 are lovely and at least in my system bested the RAM 6550. The 6550 had slightly better bass, not as magic overall.

Jim