In the broadest sense, this is a valid question in that it is theoretically possible that the amp would work. But, I don't quite understand what would be the purpose and what benefit one hopes to derive from doing this. Yes, by some miracle it might turn out that the KT88 will sound wonderful as a drop in replacement, but that would truly be a miracle (I hardly expect a KT88 to sound good even in a KT88 amp).
The reason the posters in this thread are merely speculating is because no one has attempted the experiment; no one has seen a reason to even make the attempt because it appears to be a bad idea, with only a hope and prayer of a positive outcome.
Back when we were in grade school, my sister thought it was so funny that some school science projects were based on questions or hypotheses that seemed to inquire into areas where the answer had no use or real interest; this question is the audio equivalent of the "Can guppies live in milk?" question she found so amusing.
