25 degrees sounds great.
You can do a lot of cool things with a false wall, including having a place to store all the junk you probably had to take out of the garage and haven't found a home for yet
Then again, I guess a 20x20 room is at least going to be big enough to put the bass peak down somewhere useful (28hz IIRC), and the 500 is ruler flat rather than having a ~30 Hz peak like most subs in its range so it might all work out well.
Don't worry about the sub not being able to pressurize the room; I was just trying to make the point that the extra volume would be enough to make at least a small difference.
I guess the company wasn't named after the band either ?
