Recently reread the fabulous review of the Inspire amp by 10 Audio from back in 2016. He listened to 6V6, KT88, KT120, and KT150 tubes and preferred the KT120. But the only 6V6 he tried was a new production JJ.
"I found the 6V6 tubes to have a nice warm midrange, but with noticeably limited bass and upper treble response. KT88s sound very good, although I preferred the upper frequency clarity and extension offered by both the KT120 and KT150 tubes. The KT120 power tubes sound wonderful with the 95 dB Emerald Physics 4.7 speakers, which can deliver 90 dB SPL at the listening seats in my room with only 1.5 Watts (and 100 dB SPL with 15 Watts). These power tubes were also preferred with the Fritzspeakers Carrera BEs which have much lower efficiency spec of 87 dB."
http://www.10audio.com/inspire_fire-bottle.htm
I like the KT120 very much in mine. I've tried only the Russian Tung-Sol. Powerful, defined, and yet still smooth. Used in many high-end tube amps. I've read good things about the Shuguang Treasure KT-120-Z, but I believe it was discontinued and now very hard to find.