I am sure the KT-88EH's are fully run-in now, and there are some new findings to report. Specifically, my initial impression that there wasn't much to comment on regarding differences above the bass range needs amending.
Further auditioning has made it clear what some differences are. The 6550C's sound distinctly 'creamier' on for instance vocals, while the KT-88EH's transmit more detail and nuance and generally sound less 'tubey'. The apparently higher fidelity of the KT-88EH is maintained up the frequency spectrum, where cymbals display a bit more metallic character and harmonic detail, while the 6550C's present a less distinguishable hiss that is more akin to white noise.
Some may find the KT-88EH sound less to their liking than I, since it is not overtly warm or liquid in the way that many audiophiles prefer tubes to be. But, the combination of some of the factors I have heard - better bass, more detailed articulation, more spatial specificity - have me becoming cautious with my conclusions, thinking that maybe the year's worth of prior use on the 6550C's is affecting the comparitive results more than I have supposed. I think in order to make a fair playing field that I will have to use the KT-88EH's for a year while the 6550C's sit in their boxes, and then return to the comparisons to see if my impressions will still hold to the same degree then.
What I can say is that the combo of these new power tubes, the new NOS input tubes, and the new input tube dampers - combined with me finally getting the same resistor upgrade done to the remaining speaker that was done on its pair-mate last spring (in which the underspec'ed resistor had failed completely) - has given me the best sound I've heard from this amp/speaker combo so far. I also think the Au24 SC's - whose attributes vs. the Satori are actually not unlike those of the KT-88EH vs. the 6550C - are here to stay, along with a switch from HT Pro-Silway II's to Van Den Hul The Second IC's leading into the amps. The sound is now faster and more open (and has more depth) while being less warm, but it is also less bright, and much more solid throughout the bass (that being due mostly to the KT-88EH's and the VDH IC). As a bonus, I have inklings, yet to be fully tested, that the KT-88EH's are more competent working in triode mode than were the 6550C's, and the sound of the two modes is actually closer together now in many ways, none of which I think are penalties.
Further auditioning has made it clear what some differences are. The 6550C's sound distinctly 'creamier' on for instance vocals, while the KT-88EH's transmit more detail and nuance and generally sound less 'tubey'. The apparently higher fidelity of the KT-88EH is maintained up the frequency spectrum, where cymbals display a bit more metallic character and harmonic detail, while the 6550C's present a less distinguishable hiss that is more akin to white noise.
Some may find the KT-88EH sound less to their liking than I, since it is not overtly warm or liquid in the way that many audiophiles prefer tubes to be. But, the combination of some of the factors I have heard - better bass, more detailed articulation, more spatial specificity - have me becoming cautious with my conclusions, thinking that maybe the year's worth of prior use on the 6550C's is affecting the comparitive results more than I have supposed. I think in order to make a fair playing field that I will have to use the KT-88EH's for a year while the 6550C's sit in their boxes, and then return to the comparisons to see if my impressions will still hold to the same degree then.
What I can say is that the combo of these new power tubes, the new NOS input tubes, and the new input tube dampers - combined with me finally getting the same resistor upgrade done to the remaining speaker that was done on its pair-mate last spring (in which the underspec'ed resistor had failed completely) - has given me the best sound I've heard from this amp/speaker combo so far. I also think the Au24 SC's - whose attributes vs. the Satori are actually not unlike those of the KT-88EH vs. the 6550C - are here to stay, along with a switch from HT Pro-Silway II's to Van Den Hul The Second IC's leading into the amps. The sound is now faster and more open (and has more depth) while being less warm, but it is also less bright, and much more solid throughout the bass (that being due mostly to the KT-88EH's and the VDH IC). As a bonus, I have inklings, yet to be fully tested, that the KT-88EH's are more competent working in triode mode than were the 6550C's, and the sound of the two modes is actually closer together now in many ways, none of which I think are penalties.