Here is a Fender 68 Custom  Deluxe Reverb amplifier that came in for repair recently. 
The customer felt that the amp was getting thinner sounding and had less power and clarity than it used to have. 

Since he had not changed the power tubes in over two years and was using the amp on a regular basis, he concluded that those tubes were at fault and had to be replaced. That was a very reasonable assumption and he was most likely correct about that. So he purchased a new set of 6V6 power tubes and installed them in the amp. He did not have the amp biased however, thinking it was fine to just pop those in. Unfortunately, that was not the case and the amp went down.

Once the amp was brought in for repair, we tested it and found that both power tubes failed as a result of running way too hot (under Biased). In addition, the amp itself was damaged, as the screen resistors burned out. To get the amp in order, those resistors were replaced, a new matched pair of JJ 6V6s was installed, and the Bias was properly set. So this has been a learning experience for the customer, as it ended up being a much more expensive repair than if he had the amp biased in the first place, when he changed the power tubes. 

Well, it’s all good now and the amp is working great and sounding loud, clear, and sweet.