Vintage pots have a different taper than newer pots. For example, on, say, an old Fender Blackface amp (with original pots) turn the volume pot to halfway and the pot passes roughly 30% of the signal through it. Newer CTS and Alpha pots , if turned to halfway, pass about 10% of the signal through it. Lets say the volume pot on your Blackface super reverb fails and needs replacement. You might perceive the amp as having less power because now you have to turn the volume to 6 where you might have only turned it up to 3 or 4 previously. I have found that this has a pychological effect on  many players. That is, if the volume control needs to be turned-up to 6 the amp “seems” to have less power than an amp where you only have to turn the volume to 3. What is needed here is to actually listen to the amp to see how loud it is and how clean or distorted it sounds, and try to forget about the  numbers. Set the controls for what you hear!