I know it's just a general question, and CAN be answered simply- but the matter of the fact is that there are MANY variables into why an EQ control knob is set where it is. Different amplifiers have different EQ ranges for each treble, midrange, bass, and presence control knob. You crank the treble knob to 10 (or 5 o'clock) on two different amps and get two completely different sounds. Just because one musician might crank the treble on their amp, does not mean it would sound bright, harsh, and so piercing as to cause pain in the audience. Their amp might just inherently dark, so the treble needs to be turned right up to overcome that.
A large factor is if the tone stack is located prior to the preamp valves (pre-gain EQ), or after them (post-gain EQ). My Mesa/Boogie MarkIV has a pre-gain EQ tone stack, so the treble control in particular will greatly affect the signal level overall as well as the treble frequency ranges. If you turn the treble down, the signal will be weaker no matter how much the gain knob is cranked up, thus will not become as distorted. If you turn it up, it attenuates the signal less, thus can become more distorted when the gain is cranked up. So you can see that the Treble control has a lot more impact than just making a guitar signal sound brighter or darker.
Some treble controls affect more of the high-mids, thus could sound more midrangey rather than trebly and bright. In these cases, the presence control will affect the very high end treble frequencies. You can have a relatively dark but punchy midrangey sound even with the treble cranked, if the presence is turned right down. And at other times, the presence can be turned up so high, that it gets too piercing. And what about external equalisers? They often can boost the top end way beyond just cranking the treble control.
Maybe the speakers sound shrill and harsh when heard from a distance? Maybe the acoustics of the room over-emphasised the brightness of the guitar sound, beyond what was heard on the stage? Sometimes having too much high-mids can sound harsh and piercing, it depends on the particular frequency within that range, and whether a particular amp preferentially amplifies that frequency more than other amps.
So you can't just draw the line at Treble control knob settings.
A dedicated site would be more beneficial to showcase all of the mods and original builds.
and the customer wont let me touch it,he just wanted the bias corrected,he said it sound great the way it is and didnt want to change anything in it,i think he thought the duct tape added to his sound,...thats funny ,.of coarse i abliedge the paying customer,..i'm working the site it will be up soon