With the caveat that tube performance depends on your tube amp circuitry and the neighboring driver tubes, I can give you my experience with the Music Reference RM9 mkii.
Siemens EL34s were excellent tubes that I used for years.
JJ KT77s were inexpensive and used experimentally. They lacked the “finesse” of the Siemens. I cannot give a more analytical report, since it’s been so long since I last heard them. They sounded crude in the RM9. They were more dynamic with brute force, but deviated from the tonal accuracy of the Siemens.
I also tried the Electro-Harmonix 6CA7s. They were acceptably better in terms of sounding more musical, but ultimately not what I wanted for long-term use, having gotten used to the Siemens sound.
The Shuguang Black Treasures 6CA7-Zs sounded like a good alternative to the Siemens. They are more expensive, since they were culled by Grant Fidelity. Apparently Shuguang releases sub-optimal units to meet quotas. You need a distributor or retailer that culls the factory tubes to meet higher audiophile standards. Roger Modjeski’s RAM Tubes does the same procedure for its Siemens and charges a premium for it. The Black Treasures sounded good in my amp and approximated the Siemens sound. They give off noticeably less heat than the Siemens.
I have tried the Genalex Gold Lion KT77s, and they are a less expensive viable alternative to the Siemens. I do not want to compare them to the Black Treasures, since the newer Black Treasures are sold under a different name. The GL KT77s are good and bad in that they permit me to advance the integrity of the sound of the system. They are bad in that respect because they tend to aggravate the Audio Nervosa Syndrome. RAM Tubes and others cull these tubes for a premium also.
Your mileage may vary. Generally choosing a tube depends on how far along you are in the nervosa syndrome. If you are beyond hopeless, then you will choose the more expensive spread.
