Resonant Modes & Room dimension

At the risk of alienating some, I need to pry for more info on building my sound room. I have been crunching numbers of room dimensions examining graphs of frequency modes for the dispersion ratios. I started out trying to stick to the "golden ratio" but found it too limiting. The ideal room appears to be H:10'x W:15' x L:21'. However, I live in the northern clime and need to heat the bugger. The 10 foot ceiling adds considerable heating cost. I have compared the freq. mode dispersion ratio's using an 8 foot ceing (same W & L) and see that I would be accenting the "low-mid" and "upper-mid" frequencies. But comparing to the 10 foot ceiling, it doesn't appear to be that great a difference. However, I don't know how that will translate to what I hear. I have done some reading, but can't tell from that what these charts indicate. Any thoughts?
8 and 15 are getting kind of close, no? I have 8x14x24 and love it. Maybe you could use stuffed furniture along the long sides to effectively chop up the 15, as well reduce first reflections (assuming you're putting the speakers on this wall). Room furnishings can be HIGHLY effective in quenching a midband peak. Good luck. Ern