VTL has fitted SED KT-88's to new amps on special request (it is a drop-in substitute for this application and will bias normally without modification, unlike the KT-90 according to VTL). From to what they related to me, they like this tube as much or better than the 6550C in the upper range, but still prefer the latter from the lower mids through the bass. I feel the Electro-Harmonix KT-88EH is more extended, tighter, and has more specific tonality in the bass than the SED 6550C's, but is not voiced as richly from the mids on down, and is at the same time more extended and cleaner from the mids on up, so the overall balance is on the leaner side compared to the 6550C's. (I haven't heard the SED KT-88's.) I personally would find a tonal balance somewhere between these two tubes to be nearly ideal, but given the choice, I favor the clearer, quicker, and more harmonically open presentation of the KT-88EH with most material, even if its basic character might want somewhat for richness of the fundamentals in direct comparison. But I'm something of a clarity, palpability, and openess freak, and others will gravitate toward the plusher balance of the 6550C and not care as much about giving up a little in the way of fine textural detail, 'floaty-free' imaging, or soundstage 'air'. (I prefer to adjust tonal balance at the input tubes when push comes to shove, and I also find I particularly like triode mode on my MB-185's better with the KT-88EH's than I do with the 6550C's, which seem not to thrive as much in this mode.) The phrase "less tubey" refers mainly to less added harmonic warmth and less treble roll-off along with more control in the bass - or put another way, more 'solid-state'-ish attributes, if you will.
Piezo: Although a 12AX7 might function if substituted for the 12AT7 in this position, I don't have plans to find out, because performance would probably suffer since the two tubes don't have the same electrical parameters (12AX7 is higher-gain) and the circuit was designed around the 12AT7.
