If you treat an L shaped rooms like 2 adjoining rooms you can set up subs in a way that will do the same thing as they model in a rectangular room. Nearfield subs will load that area so long as you oppose them. They use this same strategy in outdoor concerts with no walls at all. Worth a try. I've done it before. Works well.

More economical perhaps. Totally guessing on your dimensions based on pics over time.

You can build a false panel to mirror the fireplace. Other acoustical benefits there too by deleting the corners. Then your right and left channels are each boundary loaded identically too. wink

ie.

[Linked Image from imgpile.com]