I am new to tube amps , so how long does it take for an out of the box amp to be singing at its true potential
Valve "burn in" time is widely argued, mostly because some people think it's a myth. The speakers usually take about a couple of weeks to break in at low volume, and a good 20-40 minutes if you really crank it up and play.
What are the symptons of not burnt in valves?
Supposedly slightly less high end, dynamic sensitivity. This is all very subjective.