Here is the updated diagram. The width of room is a little wider than I thought, about 12 feet not counting the alcove in the bottom right corner.
I now have the sub in the upper right corner on the other side of my desk.
I tried playing some music from cds as well as from my dvd player and roku. Basically the other sources generally sound pretty good, it's the turntable that's erratic. Some LPs sound great and others have that "bottoming out" problem I mentioned before.
What are your thoughts on the use of subs with turntables? Some people think that they don't work so well together. I have a pretty big vinyl collection so this is an issue for me.