Back wall will always suck. That is where all the bass "bunches up" and kills. Also, if the sound person doesn't get out of the booth occasionally and check out the sound, the subs can be killing elsewhere while they are in a bit of a null. And from sound check to show, it can change radically.
It is a hard etiquette question on how to say "hey too much bass" when often they will say that is how they want it to sound (or if it is someone who comes through regularly, you know they know the room and so that is their desired sound). I have tried subtle, tried "uh the amps are all clipping", or "people have their ears covered" sometimes it works sometimes not.
Also, sometimes you are so busy breaking down and getting the first acts out and packed that you don't have much time to listen to the show, you were there for sound check, it sounded right and now who knows.
BTW, the new system rules.