Having now given the KT88's a fair trial I can say as follows:
Orchestral music: the KT77 wins. The sound of orcestral strings is presented in a sweet and very natural manner, to an extent that I have never heard from any amplifier. By contrast the KT88 sounds just a bit bright, as if the players had all changed to chromsteel strings (when I know they are playing composite or gut). Soundstage is superb.
Jazz: Here the slight edge of the KT88 pays off. Drum transients are incisive and fast, sometimes startling. Voices are just a tad forward. Bass is instrumental and articulate. The KT77 is a close second.
All of this is nit picking. Both sets of tubes are still quite new and things may sound different next week.
In only one area can I say one set will always excel - the KT88 has it for looks!