The ZMA has four KT66 output tubes. What is the normal expected lifetime for this type of tube in the ZMA?

I got about 2 years out of my stock set of output tubes. They were still usable when I pulled them, but noticeably less dynamic and "sharp" compared to a new set of stock tubes.

Doing the math, and knowing I sometimes play music 4-8 hours a day, it sounds like I got roughly 4500 hours out of my stock tubes, biased at 50 like the manual suggests.

My preamp tubes get swapped out a lot, so I'm unsure of how long they lasted, but I know my set were also tired when I swapped my power tubes.  

So for me, I expect to get about 2 years out of a full set of tubes before I'm wanting a refresh. If I had more money I wouldn't mind swapping tubes out every year and keeping the used ones as spares. I do use my amp a lot, and like when it's punchy and dynamic and so very consistent.

"22 months on my first quad."

Basically the same here....