which power tube sounds better: EL34 or 6550

I would agree with Rcprince's observations. I have Cary slm 100's which can use el-34, 6550 or kt88. Harmonic richness is the el-34 strong suit, especially running triode, a little tubby on bass but the midrange and highs make up for it. I only use my GE NOS 6550's when I want to rock out, gives the amp a BIG sound, much more controlled bass. Otherwise I use SED el 34's for 90% of my listening.

I have not heard a current production EL34 tube that can even come close to the midrange reproduction of the GEC KT-88s in my amp. Haven't tried NOS EL34s; since I now have a good supply of the GEC KT-88s and Tung Sol 6550s.

John tracy, I agree with you 100%. I was speaking of some NOS Mullards I've heard in my 80s, but even they couldn't match the GECs overall and in the mids it was very close between them. If you've got the scratch, they're the best way to go, in my opinion.

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.