Did you check to see how many hours the tubes had on them before you fired up your amp? Did you ask the previous owner if the tubes were original (ALL of them) or had been replaced and if replaced, from where were they sourced?
KT 150's are problematic. They are great when they are great but the QC coming out of the New Sensor factory is terrible. Few middlemen can supply reliable matched sets of them.
I apologize for being critical, but will say that as an owner of four ARC amps past and present (present are the Ref 150 SE and Ref 80S), I would have warned you that the Ref 750SE's are NOT the amps to start out with as your very first tube amps. Add the fact that they are used and your choice is even more unfortunate. The more power tubes you have, the more likely one will arc and fail. Add KT150's to the mix and well, this was not unforeseeable.
I would have taken your amps to John Rutan to check out before you fired them up for the first time. If you had done this, there is a good chance (but not necessarily) that John would have seen something amiss with bias. This concerns me; "Tube bias is controlled by two KT150 tubes, each driving a bank of eight KT150 output tubes" (from the website). With my Ref 150 SE there are four KT 150's per side but there are two bias pots per side so that one KT 150 is "slaved" to the other. When an output tube is failing or faulty, it can most often be detected by checking bias on my Ref 150 SE. Perhaps with your Ref 750 SE's the best way to check all those tubes is to pull them and test them on a good tube tester.
What you evidently did by unboxing them and firing them up right away is perform a stress test and the stress test failed. I am not trying to beat you up over this, nor should you beat yourself up about it-it is a good learning lesson.
I truly hope John can guide you on the path of getting reliable service with 32 KT 150's running in those monsters.
