Down and dirty-34s are sweet thru the mids,top/bottom a little weak.6550-muscular but lacking in top-end delicacy.88s are my FAV-fairly full range-good performer all around.No experience with the 90s.
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90s sound more like heavy duty EL34s than KT88s, even though kt90s are a drop in replacement for KT88s. The kt90 is also going to output more power in most cases than a kt88 at the same bias. I think the bass on the kt90 is much better, but that is subjective. It is definitely a leaner and more SS sounding tube than a kt88. but to me the kt88 sounds tubby. take that how you will.
EL34 - BEST mids, no question. In the right circuit, things sound as natural as you can get. Nice treble. Weakest in the lows of these tubes. The choice if you are a music lover, as opposed to an audiophile. My favorite.
6550 - Very evenhanded sound, which could be a good or bad thing. Nice if you need to back things off. Not so nice when it sounds dead/flat. Easily the most boring tube of the bunch for me.
KT88 - Maybe the best overall. Nice mids, though not as nice or natural as the EL34. Still, the mids run away from the 6550/KT90 without a doubt. Fat midbass, which I like, though it can be argued that it is not "accurate" or "capable of measuring flat". They are Number 1A for me, barely behind the EL34.
KT90 - Most rugged, by far. I got 6 years out of a quad, and 3 of the 4 could probably do another 1.5 - 2 years. Threadbare mids (not sure how it can be compared to an EL34). Probably best low frequency extension, though the KT88 pleases me more because of its lushness. Good high frequency extension. Overall, a cold sounding tube.
Thanks all for your responses. My amps are the Manley Neo Classic 250 Mono's with ARC LS-15/Ph-3. The EL-34's sound absolutely great so far. I suspect that I have not gotten the best sound yet because they don't have the 200 hours burn in. Sometime ago I changed the stock 6922's in the ARC equipment with Tungsram 6922/E88CC, and the bass improved dramatically. Subjectively, I believe this change is why comparisons with other pre amps (LS-25 MK II) resulted in only marginal improvement. Anyway, Manley does not recommend different tubes, but I wanted to hear from the community.