@rshad0000 re: KT150s, ..."I’d call it more neutral sounding. Definitely not as warm as the 120s. In my system I have warmer cables so the KT150s appealed to me more and seem to have better synergy. Both are great but different. Go with the 120s if you like them. They have a fantastically sweet mid range..."
These comments here and others help to confirm what I’m hearing too between these two power tubes. I’m re-testing KT120s again after running KT150s for the past year. "Neutral" is a good descriptive word for the KT150s in my amps too, I agree. I was yearning for a bit more of the notable midrange flavor from prior amps I had before with EL34s and KT88s. My current mono amps are designed to run only KT120s or KT150s with larger transformers and higher plate voltage.
The mistake I made the first time was not giving KT120s enough burn-in time in brand new amps with all new caps not settled in yet. Retrying KT120s now with better caps and better input/signal tubes fully burned in. This time around the KT120s have some time on them. Now, the midrange is more present, nice tones, not characteristically "neutral" like my KT150s are. The KT120s are hedging closer back towards KT88 sound a little, sounding better the 2nd time around. I think I understand a little more why some people prefer them. I like both for different reasons. Nice to switch back and forth every once in a while.