which power tube sounds better: EL34 or 6550

My understanding is that the best of the EL34-based amps might have a more tonally rich* midrange, but a less-defined bass. The 6550/KT88-based amps will sound a little-more balanced throughout the 10 octaves--that is, tighter bass, more-detailed treble, and more-than-'adequate' midrange.

I too will be interested in others' observations. I just chose a 6550/KT88-based amp for my center-channel 86dB-sensitive speaker.

* = euphonically colored?

From old memory, EL34s sound softer on top than 6550s or KT88s.

In my Jadis amps, EL34s were warm and lush in the midrange, somewhat weak in the bass, had less power than 6550s and were a little rolled in the highs. But what a midrange!!! 6550s, while not bad-sounding at all, were coarse and unrefined in the midrange in comparison, but gave more and better-controlled bass and power, the exception being if you were to find true NOS Tung Sols, which were much closer in the mids to the EL34 sound and so tended to give the best of both worlds, like the Gold Lion KT88s (my old Philips 6550s were also pretty sweet sounding too, just not as lush/euphonically colored as the EL34s in the mids). Now part of this will depend on your circuitry, but I think these generalizations are fairly valid. You should note that the Audio Matiere and Audio Aero stereo amps use a fairly unique circuit where both of these tube types are used in the same circuit, EL34 types for lower power applications, and 6550/KT88s coming in where more power is necessary. I think the designer there (who also was involved with Jadis, I believe) wanted to get the best characteristics of both tube types in one amp. To my ears, it's a very successful design.

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.