From the ARC REF 6SE manual:
The 6H30 tubes in the Reference 6 should have an expected life of approximately 4,000 hours, while the 6550WE tube in the power supply should have a life expectancy of approximately 2,000 hours.
That said, I demo'ed a REF 6SE with 5000+ hours on its tubes and it sounded marvelous. I wouldn't necessarily recommend this, YMMV.