i usually dont have to convince anyone of anything because people coming in to work with me are highly impressed with my previous work
sometimes they are shocked at how i get there when im like, ok plugin, ok record. sure im kinda like a tone factory and im not really innovating the sound of the guitar, but these bands really arent innovating metal or song writing either....
but im rambling... at any rate i am well prepared to show the client the difference between their amp and mine. its a pretty massive difference, honestly.
occassionally i get the dick head who thinks he knows everything (and im not claiming i know anything at all) who wants to use his shit. in those situations i have to try the convincing methods, but if the client disagrees with me, than its his shit record, not mine! and i take my name off of work i dont agree with, so i wont be associated with it anyways.
sounds shitty i know, but there are just dudes out there that would rather walk home with a shitty sounding cd and be able to tell people, "yep thats my amp you're trying to hear there.."