Not necessarily. Swap the output valves to the other channel, see if the problem with the sound follows the valves.
4 years in what I'm assuming is a PP amp could easily mean they need replaced, that's a fair amount of hours.
“Music has always been a matter of energy to me, a question of fuel. Sentimental people call it inspiration, but what they really mean is fuel. I have always needed fuel. I am a serious consumer. On some nights I still believe that a car with the gas needle on empty can run about fifty more miles if you have the right music very loud on the radio”
Hunter S Thompson