Tweaks when building a listening room?

Instead of 2 x 1/2 inch drywall, I suggest 2 x 5/8 inch. I also strongly suggest gluing the drywall to the studs & between the layers,in addition to screwing. Tape and mud the 1st layer before applying the second on an offset of course. The difference is not subtle and the cost is not prohibitive. Install soundproofing, tape and a trim piece along the gap where the drywall does not touch the floor. I would want a spare 20 amp line as well as normal 15 amp outlets per code.
I envy you-have fun.

If you need isolation of the sound from adjoining rooms, I suggest you read the information on the Green Glue web site.

http://www.greengluecompany.com/

Their testing indicates that two layers of drywall are essentially no better than one when trying to reduce sound transmission through a wall. The articles on this site are very informative.

Very...like its not the thickness or fastening,but the pliability.I wonder if 3/8" would nt be the best as it would bend easier and thus absorb more than any other???

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.