Your thoughts on these different tubes?..

I,ve owned two tube amps recently from the same manufacturer, Rogue. The Stereo 90 uses KT88, and I loved the authority in the bottom end and the headroom of 90 wpc. I have the Atlas now, and the el34 gives a fantastic 3d sound. A beautiful midrange through my Nola Boxers,. The bass is a bit lacking, but I'm using a stand mounted 2 way speaker.

If you have an amp that will use a variety of these tubes, it won't be a true reflection of each various one, from another number family.

IOW, an amp that can use a KT88/6550 type, and an EL34/6CA7 type, can't reflect the true sonics of each type.

Switching brands of each type can make your amp sound totally different.

The overall design of the amp will have a great influence on the sound also.

In general, the KT88/6550 do have a more detailed and dynamic sound, and can almost sound like a solid state in the right amp.

The EL34 is more euphoric, and seems to focus mostly on the midrange. It can still give deep bass, and extended treble.

With all things being equal, the 6SN7/6SL7 is considered the best by most. The 12AX7/12AU7 a close second best.
These two are similar, but the 12AX7/12AU7 was made smaller, to fit the miniature tube bottle, and socket. The 6SN7/6SL7 is the older, larger octal type.

The 6922/6DJ8, the third choice by a lot of audiophiles who have had a large variety of tube amps over the years.

By the way, the 6DJ8 was originally designed for TV tuners and can work in circuits to produce a ridiculously broad frequency response. An amp that uses the 6DJ8 in conjunction with the KT88 can fool you into thinking you're listening to solid state. I find that the 6DJ8 with the EL34's keeps the warm mids with increased bass and highs when you compare it to the 12AU7.

I think el84s may be the best of the bunch, if you can live with 15-30 watts,

I've had a Rogue Cronus Magnum with the EH KT90 to my ears a transparent tube with good definition top to bottom. For me the best new issue KT88 is the Shuguang Black Treasure - dynamic with punch and excellent across the frequency range. Likewise, if you have an amp that takes 6sn7s and the amp allows for swapping in cv181s might want to try the Black Treasures as well. Like the sound of 12au7 , especially the NOS CIFTE Mazda. Have found them more dynamic that NOS Tungsols & Sylvanias that I have. Only tried El34s once, found them lacking beyond the mid-range, prefer the KT88s.