most tube places burn them in for 40 hours before they test and match them. if a tube is going to fail its in the first 40 hours and to get the best test results they need the burn in.....but check before you buy....pre amp valves last ages....power tubes???? I change mine every 10 months in my main amp, you should also change the phase inverter when you change your power tubes. I don't think leaving your amp on stand by overnight will burn them in anyway so I wouldn't worry, it might help but I doubt it. hope that helps...have a good one.