Your thoughts on these different tubes?..

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.

Facten - I agree about the non-mid range response of the EL34; however, I have an original circuit design ST-70 (upgraded every component/increased capacitance, but didn't alter the circuit) that outperforms all of my 200W/ch SS gear in the bass and highs. In general though your thoughts on the EL34 are correct.

Also, the Shuguang Treasure are great.

IMHO...it all depends on the speakers you're running, amp design, etc. In my particular case, when comparing EL34-based monoblocks to 6550-based stereo amp by the same manufacturer, using very similar design...I preferred the 6550-based amp.

It didn't come down so much to performance at the frequency extremes - it was more that the EL34's midrange occasionally sounded grainy compared to the 6550, and that dynamics seemed compressed with the EL34's. This is all partly due to the fact that both amps use 4 power tubes per channel, so the 6550 just has more power (and my speakers are able to do something with the extra power) - but it also comes down to resolution IMHO. The 6550's just seem to be higher resolving to me.