What i would say to most bands is that if you don't know the pa systems well, just leave it to the soundman. Explain what type of sound you are looking for, then just let the soundman do his job. Don't try to take the lead, because your job is to perform well and not be affected by the sound too much.
And if you know pa systems well, come early to ensure the sound you require. Work alongside with the soundman, don't think you know better and act so proud. Remember, the soundman will be there when you are performing, he is there to monitor any feedback or sound issues while you are performing. If he is ill-treated, he might go for more coffee breaks or don't do anything since he finds you too particular.
Main point is, always respect the soundman, take him like another member of your band, his task is to do your sound. Feedback to him the type of sound you are after, the type of band you are. Even if you think the soundman isn't that 'top notched' guy, still work alongside. Remember, many times the soundman will not put maximize effort on your band, not because he cannot, but because he is probably already tired out with so many other bands and jobs to do. If he is in a good mood, he will tune your sound very well. So learn to please the soundman!