I've tried GL KT88 (stock tubes), KT120, and KT150 in my VAC Signature 200iQ monos. The KT150 were the most recent acquisition - and they were terrible. Extremely powerful low frequencies and gobs of slam - which honestly was fun - but it completely swamped the rest of the spectrum and was a very unbalanced sound.
The KT120 - I like them a lot. They're not as polished and refined as the GL KT88 in this amp, and so I think most will prefer the 88's here, but I also think the KT120 are at least competitive here (unlike KT150). I'd gotten so used to the KT120 sound over 10 years of use in Rogue Apollos, that I think it's been hard to wean myself off them in the VACs. They feel like home base lol. I'm currently, maybe, transitioning from KT120 to GL KT88 in my VACs.
I'm sure the KT150 performance is very amp dependent, and I believe they'd be fantastic in an amp properly designed for them - but not in the VACs.