Tweaks when building a listening room?

A drywall surface on studs resonates with acoustic energy just like a drum head. You need something to damp that resonance, or the whole wall vibrates and transfers sound into the adjacent room. Adding another layer of drywall adds mass, which lowers the resonant frequency, but doesn't lower the sound transmission much. According to Green Glue, adding this product between the two layers of drywall provides the needed damping and the test results show a dramatic improvment. I tried this on the ceiling of my dedicated room and it worked extremenly well.

The green glue is very worthwhile but there are other types of permamently resilient glue that are also similarly effective.
The reason for the 5/8 drywall is that the added mass with the resilient glue helps the low frequency isolation which has always been most problematic in my experience.

Don't allow any 90 degree corners. Use dry wall to enclose all the corners to make bass traping. Use the new sheet rock called Quiet Rock I believe. You won't have to double it probably. Use rock wool as insulation. Wire at least one home run per component and then add more. Install a separate electric panel for your room where all audio/video circuits are located.
Build in the design a wood floor. If concrete, build a wood floor over it with 2 by 4 laid flat, then plywood, then carpet.
Use Cardas Golden Ratio Data to determine the best size room for your needs.
Hire Rives Audio for overall room design. As for book cases being diffusors and Traps, go to Real Traps.com and watch the video on diffusion. In fact it would really help you to view all their videos. While you're at it go to the ASC site, readytrap site, and rsc. sites.
I could add more.

I built my room about 3 years ago. If I could build it again, I'd include the ideas I've outlined here. I've learned so much since I built my room originally.

Have fun,
Steve

The flexible conduit mentioned above is a must. You always want the chance to pull wires. also, run an empty conduit with a string inside. you will be amazed how soon you will be using it.

On the walls, I would use two layers of sheetrock, but of different widths. I would probably put 3/8 against the studs and then use that newer fiberglass-based mold resistant sheetrock as the outer layer. This stuff also does not vibrate like regular sheetrock. I stumbled on this fact when I mounted some in-wall speakers in both kinds of sheetrock and the fiberglass stuff did not vibrate as much when I touched it while playing music. One other thing you might do is tack a layer of accoustic rubber between each layer of sheetrock. This stuff stops soundwaves really well.

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.