FWIW, I am running Mullard reissues in my McIntosh C220 for over 10 years with no issues. I run it about 20 hours a week, so that's about 10,000 hours so far. Tubes tested strong a few years back when the preamp was sent to Audio Classics for a repair. However, this is a hybrid design, so maybe it’s easier on the tubes.
FWIW, when I asked a Mac rep how to tell when I need to replace the tubes, he said "your preamp will sound broken", whatever that means.