Your thoughts on these different tubes?..

Short version. Output KT88s are more detailed and more powerful. EL34s are warmer and more euphoric especially the midrange. My amp is not bass shy with EL34s due to massive iron, I believe. I do not doubt as the previous poster stated different tube manufacturer's output tubes will sound different among the variants. My experience is limited to Ruby KT88s and Sveltalna, JJs and EH EL 34s. Personally I like the EL34s better in general in my amp which will take either and I listen to more jazz than anything else. If I listened to mostly classical or rock I might opt more for the KT88s. On the driver tube front, I believe the manufacturer absolutely has more to do with sound differences than do tube type. Of course, YMMV.

I f have found that the design of the amp has more to do with it than the particular tube. I have two great sounding amps, one has 6sn7/kt-88's and the other has 12at7/6ca7's. Both are equally musically and emotionally involving. This is true only for push pull amps Single ended is another story

I tend to enjoy most of the circuits I've heard that were designed for (and use) EL34's with 6922 or 12AU7 driver boards. However, it really comes down to your tonal preferences, the efficiency of your speakers, and which designs will work best in combination.

I'd suggest you try a preamp that uses 6SN7 tubes. It probably will be a more full sound than with a 12AX7 type -generally-, many exceptions. There are still many available and affordable NOS tubes to have fun "rolling". The 6SN7 are just a great tube "family".
Your speaker's efficiency will dictate your power requirements but KT88's/6550's are powerful enough not to limit you too much, allow for tube switching fun and decent bass. (Quicksilver amps are a good value choice here.) I find EL-34's to tend to sound thin in inexpensive amps. Do try EL-84 based amps if you have more efficient speakers. They'll surprise you!

I prefer the EL-34 over the KT-88. In regard to the 6922 as opposed to the 12AU7, although they are two entirely different animals; I like them both. My impression of the 6922 is of a very neutral, high definition tube that can create a 3D soundstage; while a 12AU7 is warm and beautiful, it sounds best with female jazz vocalists.