I should just mention something before we go too far with this... if you just switch off the computer and never come back your HT dimensions and room acoustics will be as good as or better than the rooms of most of the board members, probably including my own.
Just try to keep the dimensions from being exact multiples of each other, and remember that no matter how bad your dimensions are bass trapping will reduce the effect of the offending room resonances.
Everything we talk about here is to give you an idea of things which *could* be considered during construction which will give you a head start on the inevitable tweaking, treatment and re-arranging you will probably be doing for the next few years
