room shape

Actually, you can follow some basic concepts from speaker cabinet design books!   The idea is to keep non integer relationships between the six walls.   This way no given standing wave is re-enforced.  Note that solid walls reflect bass while the bass will pass through less solid walls, such as sheet rock and OSB.   If the bass can pass through you are less likely to set up a standing wave in the room.   Likewise, dead rooms tend to kill non coherent musical distortions sooner than music, which is more coherent.  

If you are really serious about this, talk to a professional sound engineer or architect who specialty is designing sound rooms.   They will know what materials to specify and how to design the room to make the most bang for your buck.     

BTW, one thing I forgot to mention is in a larger room you need speakers that can comfortably fill it with sound.  That is, they need to play more loudly with the same clarity and definition that smaller speakers would play in a smaller room.  

I am going to guess that most of us probably have speakers that should be in larger rooms than we have.  I am certainly guilty of that.   If that is correct, then perhaps this is not as much as an issue.