Converting two-car garage to listening room. What would you do?

My wife is angling to get my mountain of audio gear out of the living room and I have graciously consented. :-) So off to the two-car garage I go, but first it has to be made livable, and we've started talking to contractors. It's not a huge space: roughly 18 x 19.5 feet, with a vaulted ceiling that crests at about 16 feet. I'll be leaving the basic structure in place but everything else is up for grabs: I can play with dimensions (a bit), lighting, where I put the (20 amp) outlets, acoustic treatments, and so on.

My main question is currently if I should "toe in" the walls so as to not make them parallel. Would it help the acoustics if I had four walls that measure, say, 18 feet and 17 feet on the shorter sides, and 20 and 18.5 feet on the longer sides? In that case, no corner would be at precisely an 90-degree angle. I read somewhere that that will eliminate or reduce room nulls and peaks, but I don't know much about this stuff. However, I'm willing to learn! Any acousticians here? Words of advice? Thanks!