Personally, I wouldn't get too anal about the supposed differences between EL34's and KT90's (or 88's, or 6550's). IMHO its far more about the design of the amplifier these tubes are used in. One of the limiting factor of EL34's is that they produce roughly half the power of a KT90. Typically, an amp with 4 KT90's per channel will produce about 150 watts +/- and one with 4 EL34's will produce 75 watts +/-. Once you have picked an amp which will drive your speakers properly you can fine tune its sound by tube selection, and you will find differences between the types being discussed. FWIW in an amp which is designed to accept both EL34's and KT88's, KT90's and 6550's, I would agree with the general statement that the EL34's will have the smoothest midrange and sound weakest in the bass. The 6550's the most neutral, the KT90's the more "detailed" and the KT88's (which I use) a tad warmer and more airy than the former. But it all depends on the amp and, to a great extent on the small tubes used as well in conjunction with the power tubes.
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?