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.

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.