Tweaks when building a listening room?

If you want the room to sound entirely neutral, avoid drywall entirely. Sound bounces off drywall, distorting the music as it reaches your listening position.

There are sound-absorbing wall materials that can render the room anechoic, and this will enable you to hear your system without participation by the room. You can turn your system up as loud as it will go, and you will be amazed at what it sounds like without echo in the room.

However, these wall materials can be difficult to work with, and may not be aesthetically pleasing, so you will also need to figure out some means of covering them that does not then bring back the phenomenon of sound bouncing off walls. If fabric wall covering can work for you, that might be best.

Merely putting the sound absorption material inside the walls, and covering them with drywall, will not do much good.

The number one rule is "broadband absorption' - keep the acoustic treatments even. If you have floor to floor carpet then you may already be ok in the mids and treble from the get go. Then all you need is large thick bass broadband absorbers and plenty of plush furniture. See my virtual system example for what you can do with a fireplace. There is such as thing as too much mid and treble absorption - so keeping the room lively may actually make it sound better. It is pretty much impossible to overdo treatments in the bass - so do as much as you can aesthetically handle...

lots of good advice and ideas.

i would recommend getting some professional advice on the effect of the shape of your ceiling on the sound. not all 'cathedral ceilings' are the same.....the solidity of the structure and the surface materials will have an influence as well as the exact shape. with a 15' width and height it might be a problem in terms of room nodes.

fixing it later will be expensive and a pain.

the room has the potential to sound very good. a purpose built 2-channel listening room is a joy.

Building the room with the ideal dimensions will make the biggest difference. In your case installing a drop ceiling to the right height would be more ideal. Check out this web site http://www.decware.com/paper36.htm for the golden ratios for room dimensions.