- the size of the room
- is the room full of people or empty?
- are you outdoors?
- have the speaker cones been in a humid environment and absorbed some moisture?
- is your amp coupled to the stage, sitting directly on it?
- with some speaker cabinets, where are they pointing?
- How old are your strings?
- etc, etc
I got a callback on one repair that I did saying that the amp now had more treble. What had happened was I had taken the treble knob off the amp and put it back on in a slightly rotated position. This person was locked in to the notion that their treble setting was "7" and to their credit their ears were good enough to hear the difference. They could have adjusted it it by turning the treble down to 6.5, but it wasn't right for them until I pulled the knob off, unrotated it a click and put it back on. I guess there's nothing wrong with that, it just makes me say "hmmm..."