[LoKi]

It's fine. Tubes are only bad when they're blown, or starting to sound bad. Even when they are sounding bad, they are still functionally ok, it's just a matter of preference for tone. You can tell if their dying by tapping on each one with a pen lightly while the amp is on. If you can hear the tapping really loudly through your speakers, they're on their way out.

Standby is just the plate warmers on, so not a lot of voltage is going through the tubes, that's why it only happens while you're playing.