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.
