Generally the 90 is supposed to have more punch and the 88 a tad more delicate in its dissolving powers.
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Depends on the circuit they're in. In my JA80s, the KT90s were thinner harmonically, brighter and lacked the dimensionality of NOS KT88s; punch was about the same, although the brighter sound of the KT90s made the high-frequency leading edge of transients seem a little "crisper". The 90s seemed cleaner sounding and more detailed at first, but got duller and more wearing quickly, while the Gold Lions have only gotten better and sound more "right" to me. Can't tell you how the new KT88s stack up, and keep in mind my amps are older and were designed around the KT88s; newer designs from Jadis, such as the JA 100, 250 and 800s and Orchestra Reference, were designed to work best with the KT90s.
"...PUNCH was about the same, although the brighter sound of the KT90s made the high-frequency leading edge of transients seem a little "crisper". The 90s seemed cleaner sounding and more detailed at first, but got duller and more wearing...etc"
I do not know how can you, or anyone else, tell such a subtle difference? I wish i have such a delicate hearing as you.
What is "punch?"
I'm thinking of dynamic impact on transients. It's really not that subtle, honest! An orchestral climax can include all instruments in all frequency ranges. Some equipment might emphasize the higher frequency portion of the sound more than the rest of the frequency spectrum when reproducing the transient. The result may be a crisper or sharper-sounding leading edge of the transient, which can sound exciting but ultimately can lack the weight of a transient reproduced evenly over the full frequency spectrum. An example I can think of was my early Krell SBP 64x DAC--when I had it upgraded years ago, the first thing that struck me was that orchestral climaxes had more weight than before and seemed less grating on my ears; Krell told me that the upgrade had smoothed out some distortion in the treble they had found, and that was the result. Another example I can think of is the difference between early Krell and Meitner amps in reproducing orchestral climaxes--the Krells had much more apparent high frequency energy than the Meitners. The KT90s in my amps had more of a high frequency emphasis than the KT88s, which seem to me to better balance the frequencies (that's also why I use Telefunken 12AU7s in my DAC instead of the Mullards supplied by Audio Logic). I think that if I listened more to rock music, I might prefer the KT90s in this regard, as they would emphasize rim shots, cymbal crashes and snares. Make any sense?