Welcome Joe,
I think your choices will be fine but for me, I would want something a little bigger for the mains. Just personal pref.
If I may, what will the rest of the basement be used for? The reason I ask, is for the layout. The way it is set up now, the stairs come out right into the back of the theatre area. To reach ANY other part of the basement, you would walk right past people watching the movie. This would drive me insane. (My 3 kids can't stay in one area for more than 5 minutes)
If it were me, I would turn everything 90 degrees and have the screen on the wall adjacent the furnace room. That way when people come downstairs it's into an open area that won't disturb the movie watchers.
Then, I would then put stairs directly off the landing into the laundry room. (The stairs would form a "T" at the bottom) This way, you could turn left to do laundry and not disturb ANYONE in the bsmt.
As far as the sub goes, why not put it under the stairs with a grill cut-out? Or even in the furnace room?
