If you have the chance to actually build the room, try to avoid symetrical walls. A room with 5 walls is, acoustically, miles better than 4 room. For early reflections, the most walls, the better. In fact, having a fairly square room can be improved by adding diagonal "walls" to cut the corners. An octagon.
If there's the chance, the ceiling shouldn't be paralel to the ground. Even a 5º angle is better.
Now, for the construction itself:
Low end needs MASS to be stopped. Mass + Absorptive material for low end absorption. Really dense wood is pretty good, but nothing beats glass wool and fiberglass insolation.
(I'll continue in my next post. Need to check some numbers)