@hilde45 My friend has 3 year old M120 amps. They can be used with KT88 tubes.
Best to check with Mike @ QS and give him the serial # of the amps, and reconfirm to be sure. The latest version of the Mono 120s with the large transformers in the past two years may have a different recommendation. Not sure if it's plate voltage or what changed, if anything. Or, maybe something was learned since. I ran KT120s and KT150s in my Mono 120 amps, no issues. In fact, originally I tried to order them with KT88s. No-go. On three separate occasions while checking on internal biasing of the small tubes, I asked and pressed it with him pretty good asking more than once. Mike told me no, "not" to run KT88s, that I'd, quote: "burn them up" in my Mono 120s. His web page has been updated since then. KT88s are removed as an option for output tubes now for the QS Mono 120 amps. Definitely worth re-checking. Best of luck.
Mono 120 now shows: "Tube Complement1 12AU7 input, 1 12AT7 driver, 2 KT120 or KT150 outputs.