You could go with an in-wall sub, or hide the sub under a coffee table or behind a couch, but I wouldn't put it behind a door in a closed closet. At the end of the day, you'll probably just have to experiment.
I have to pull them out of the way when using the system. My better half picked out the curtains.Are the curtains acoustically transparent or do you pull them further back when you are using the system?