Last time I checked, taking charge and setting boundaries was being professional. Letting yourself be pushed around by all the demanding musicians you have to deal with wouldn't be professional. What's not professional is encountering a situation or requirement that you don't like and letting yourself get all bent out of shape about it. The ability to adapt to the situation and find a solution or compromise that works is professional. The ability to adapt and overcome is professional. Getting annoyed because a soundman who has to manage the sound for an entire festival with thousands in attendance isn't catering to one musician's request isn't professional.
This is where people's careers start to slow down and hit the wall. Being flexible and easy to work with will give you a good name in the circles that matter. Being "that guy" who made an issue out of using his own DI box at a large festival will give you a bad name in the circles that matter.
Besides, where does this idea come from that someone who makes more money is somehow supposed to behave more professionally than someone else?
Last time I checked, the more money you get paid the more responsibility you have. Usually with more responsibility comes more authority. So, if anything, the soundman is running that part of the show and he has some right to tell the bands what to do and how (in regard to sound issues). That might not be the way it is but the bottom line is whoever is paying this guy, is his boss. If this particular band is not signing his check, then he doesn't work for them...he works for the festival.