Converting two-car garage to listening room. What would you do?

Hey congratulations on your wife and thanks for the super helpful contribution.

Don't worry about parallel walls. Worry about overly similar dimensions. Sound travels in waves. These reflect or bounce back and when they do they either cancel creating a dead spot or reinforce creating a really loud spot. Where they null or reinforce changes by wavelength. There are books you can buy like Robert Harley's Complete Guide to High End Audio that take you step by step through the process of figuring out where they will be at different frequencies in your room. 

Since sound travels across all 3 dimensions then they will be doing this same thing across the height, width, and depth of the room. In the worst case you have a cube, all 3 dimensions are the same, and so certain frequencies will be really, really bad and this is very hard to fix any way other than moving the walls.  

Yours is pretty close to a cube. There's a neat little exercise in the book you can use to visually plot out where different frequencies will reinforce, essentially telling you in advance where the big bass lumps will be. I would highly recommend doing this ahead of time because however hard it is its way easier and cheaper than trying to fix with a lot of tube traps later on.  

Probably it can be fixed very easily at this stage by simply framing up one wall to make the room a little narrower and more rectangular. At the same time this wall can serve multiple functions. It will be a huge sound barrier, either from outside or to the rest of the house. And it can also be designed to function as a super effective bass trap. You could learn and do all this yourself but this is where a lot of guys will want to hire a specialist. You haven't mentioned budget. This stuff can be as reasonable or expensive as you make it. 

The practical solution would be to leave the garage door in place (for outer appearance and for the future) but seal it up real good, and build the additional wall inside of it, for inner appearance and outside noise abatement. 

If you care about power then bring one - ONE! - 20A line direct to wherever your equipment will be. Put lights and non-system outlets on another circuit.

Unless you are willing to pay for professional design, and maybe even if you are, I would highly recommend you buy and read and study that Harley book. You will be shocked how much is in there. In a week or two you will know more than 90% of the guys on this site. And half the professionals who are happy to take $100/hr for what Harley will teach you for $20.

Get earphones and a dedicated amp

When you get set up and are adjusting your speaker positioning.  I recommend you listen to the great music by the Cars.

Build a rectangular room interior to the garage having length 1.9 times and the width 1.4 times the exterior wall height. Put a ceiling on this room.You would not have to remove garage doors and would have storage space between room and garage door and space above ceiling and the roof. Check out best dimensional ratios for audio room according to M.M. Louden.