Hello. Symptoms may include all or part of the following;
decreased frequency response, for example it sounds as though highs and lows have been cut.
Your amp may also not be as loud at a particular setting as it was before;5 or6 might sound like 3.
Mushy bass response.
Some tubes might become microphonic and cause whistling or feedback.
I've found decreased dynamics and lower volumes at settings which were normally much louder to be the most common.