Think of the KT90 as a 'heavy duty' EL34. A KT88/KT90 tube will have more bass than an EL34, as others have mentioned. They will also have more power, FWIW.
Tube amps for classical music FU29 vs EL34
I'm not familuar with the sound of different tubes/tube amps. I have a a Jadis intergrated has 4 KT90's, and has a good sound, but the speakers limit the amp. Soon to be rectified, new speakers with a 87db rating late this summer. But I'd like to get another amp for primary system, keep the Jadis for a second system. I'm looking at 2 tube amps, one has 4 EL34's the other 4FU29's, both intergrated amps. I'm guessing the EL's will sound like the KT90's, and have more bass end. But with classical, I can forgo some "punch". The FU's look like it may offer more complexity in the mids, for orchestration and vocals/opera. Right track?