Did they tell you what kind of problems some were experiencing?
I've been running KT120s in my Rogue M-180 monos and find they run so much cooler than the KT88s did in my Cary CAD-120S Mkii which ran quite warm. I'm not knowledgeable enough to know if that's due to tube design or amp architecture, just relating my experience of never having a problem with the KT-120s. I asked Cary about putting KT-120s in the CAD120S Mkii and was told it would be a great match, but I never did since shortly after I moved over to the Rogues. Since the folks at Cary have also told you it would be okay, my advice would be to put them in, do some listening and see what you think.