I'm not following you on why 24/7 does not equate to tubes. Traditionally, it has been thought that it's the turn on / off processes that limit tube life, and that most line level tubes (we are talking preamps here, no?) will last for a long, long time if running constantly. I've been using tube equipment for over 30 years and I have never had a line tube fail. I may have changed them out of OCD, but nothing ever failed.