Definitely replace well before failure. There are many horror stories of the effects of tube failure in ARC amps. One of the best things about the VTLs I own is how well they handle tube flame outs, taking down a fuse only with lots of on board diagnostics to help you problem solve the issue
ive also found that pre amp tubes also need replacing well before rated life, they gradually go off and sound much more wooly, in my ARC phono and pre I found that even at 1500 hours they were markedly worse than a broken in new set
ive also found that pre amp tubes also need replacing well before rated life, they gradually go off and sound much more wooly, in my ARC phono and pre I found that even at 1500 hours they were markedly worse than a broken in new set