To @vlad427 , I have those same (the largest) transformers in my QS Mono 120 amps, and mine were made to specifically run KT150s or K120s. Designed with higher plate voltage to run these tubes only. Unlike you, I cannot run KT88s in mine Not for long, it will burn them up according to Mike. If he told you to run KT88s, there is a reason for it, and likely the KT88s will/can run "in their window" of optimum operation, bias, plate dissipation on the mark with your specific version of the V4s. While his answers are short, no less meaningful. The advice is solid, sometimes he does not share all the reasoning or details behind it.
If you’ve been running EL34s in the V4s for a while, its possible you might have gotten use to how they sound, maybe a bit lighter weight with slightly less drive and less impact than KT88s - with your particular speakers. I track them and just re-checked, still seeing no great hope for quads of KT120s or KT150s from sovtek Russia. The war. I keep quad spares of 120s, 150s,, and a backup SS amp to run to rest my larger KT tube amps, if it helps to know. It’s not worth it right now. If I could run KT88s in my amps right now, I would. They can sound really nice, decent kick. Notably less cost.
Do you stil have your former KT88 tubes, or have you checked out other new sets of KT88s lately?