Also use the braces as a place to put R11 or R13 spanning the cross-section of the cabinet. You want insulation to cover three-sides of the cabinet's interior, and in large cabinets you also want to break it up into sections seperated with sheets of insulation spanning the cross-section. It does a great job of trapping unwanted midrange reflections but bass will pass right through.

I have considered braces utilizing threaded rod and turnbuckles. I wouldn't think you would need very large rod because of the strength of the steel. You could use the turnbuckles to provide the exact amount of pull or push on the opposing panels. With the small amount of raw materials used you shouldn't have to adjust much for the displacement. The only thing that I think to consider and have discussed lightly with Wayne is resonance of the steel. I think we decided it to be a non issue. Just more food for thought. Jerrod