I had good success with the walls tapered,the room is somewhat narrower from the speaker end, wider from the listening end, high ceilings..... and sloping upward from the speaker end of the room, towards the listening position.
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First thing is to get your dimensions right. Golden ratios are a good start (see Alton Everest's Master Handbook of Acoustics), but they don't really tell the whole story. There are often better ratios depending on the volume of the room. We look at each room using 3 different programs to examine mode spacing, total modes, axial mode build up, tangential mode coincidence, and bonello criteria. For most, using the ratio's is much simplier and can lead to reasonable results. But the dimensions are the most critical element for a couple of reasons: they can't easily be changed once the room is built, they control which bass frequencies are accentuated--and if the bass is wrong--just about everything sounds wrong.
Second is to then determine the geometry. Angled walls are good for large spaces in a dedicated listening (2 channel) room. They allow you to have a live sound but get rid of slap echo. They do not change the modes significantly because the angles are relatively small compared to bass frequency wavelengths.
Then it's time to work on speaker and listening position followed by acoustical treatment. This is the area that can really get quite complex and is very dependent on listener preferences. What volumes do you listen at, what type of music, what characteristics do you like (reverberant sound field vs direct sound only--most audiophiles lean slightly towards the reverberant field as it provides better spacial cues.)
As to HVAC, Everest's book can help you there too. It's generally a good idea to have a plenum separating the duct work for a dedicated audio system and then use long linear ducts so the air flow velocity is slower and thus much quieter. Returns have similar issues.
Lastly, you may want to take a look at our site for additional tips. In particular the listening room, which is a basic tutorial on room acoustics.
I would have to concur with Nrchy. There may be lots of info on the net but, short of making acoustics a hobby and investing time & money learning & experimenting (which can be fascinating), you'd probably be better off (or cheaper off) investing in professionals such as Rives & co. Let them worry about the room!