Your thoughts on these different tubes?..

My impression of output tubes. First I have say to with any of the tubes different manufacturers of the same tubes all sound different. An example a SED KT88 sounds different than a Gold Lion re-issue and EAT KT88 is even more different. Just comparing KT88's the SED sound very warm Gold Lion picks up more detail and the EAT has even more detail and a lower noise floor. The only tubes I have from the same manufacturer are the SED's. KT88 very warm with a little more bass than the 6550C's which are very linear. The EL34 has a mid range magic but lack bass and power. My KT90's are Ei KT90 type 2 and is more forward sounding in the mids with good bass but a steely sounding high end and the most power of them all. I haven't heard or tried the EH KT90's so they most likely very different. As far as 6550's I have 3 different quads. SED's and Tung Sol original solid grey and black plates. Even with the Tung Sol's the black and grey plates sound different. My favorite of the 6550's are the black plates with the grey's being very close. The SED's in my opinion are not even close to the Tung Sol's. None are bad mind you.

That's the long version and the short version is you really just need to try them.

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.