I appreciate the do-it-yourself attitude but my recommendation to you if you have never performed any type of electrical work is to hire a professional. I realize the temptation to do all renovation work yourself because of the cost savings and the assurance of getting what you want the way you want. But in the case of electrical work, especially with the complexity of work you are looking to undertake (which involves installing boxes, running wire, installing all outlets/light switches and fixtures, tapping into and installing new circuits in you circuit breaker box, and then you have to make sure everything is wired correctly), this is not only potentially dangerous but potentially lethal if you are not sure what you are doing. I deal with renovations and new construction all the time with the business I'm in but with the exception of replacing fixtures, outlets and light switches I will typically hire out for more complex electrical work. There may also be specific electrical codes in your area that will dictate certain installation methods and/or materials.
With that said, you are installing quite a few dedicated lines. One thing to be conscious of is when you have audio equipment plugged into different lines (even dedicated lines) the possiblity exists for having ground loop problems if all your equipment is connected to one piece like the amp or preamp. I mention this because I had two dedicated lines with video equipment connected to one and audio to the other but the system was plagued with a ground loop. Now I have everything connected to one circuit which eliminated the problem. But hey, it's relatively cheap to add them now and if a ground loop is a problem later it's as simple as plugging everything into one circuit.
One layer of 5/8" drywall is good. Two layers of 1/2" drywall would be great. Stagger the joints and glue between layers. Solid. I can't imagine the textured finish on the ceiling and walls making that much of an acoustical difference. I would go with a smooth finish just based on looks.
If any of your walls are exterior and noise transmission from street to listening room or from listening room to neighbors is a potential problem be sure and add insulation to your walls, not just your ceiling. For 2x6 walls you will want to use an R19 thermal insulation with vapor barrier between studs and drywall. If you want to isolate walls from other rooms in the house add 3" acoustical batts between studs.
Completely rewiring and renovating a 12'x27'x9'-6" room to a dedicated listening room with four dedicated lines is a big job. There is a lot to know. And I'm totaly envious. I would be tickled to death just to have a separate listening room instead of sharing duties with the living room. In any event, good luck with your project. Sweat equity makes the finished project a lot sweeter.