I am by no means an expert but based on my experience two tubes might reach their optimal operating point a few seconds apart. as in, it is possible that when you turn on the amp the two channels have different ouptut levels and after a few seconds the imbalance goes away. this is not just due to the tubes themselves but also due to the circuit requiring a few serconds to bring the tubes to the operating point. but I have never seen or heard about this taking more than a minute. usually 15 to 30 seconds. in your case you already know that it is the tube that is the source of the problem so if you already gave it a good amount of playback time and the problem it still there, id say send it back and get your money.