Thanks for the reply Dr. Hsu!
I'm not sure I would want the sectional (yes, we've already bought it) 18" away from the wall from the rear wall. The only way I could possibly swing this is by removing part of the wall. Take a look at the 2nd picture. You will see that I placed the rear wall around 13" away from the foundation wall, to clear to sewer pipe (didn't want a soffit). The three cutouts are for built-in cabinets. I have the same depth clearance of 13" at the floor. I would have to build an alcove at the floor, placing the sub in there sideways to have it fire into the rear of the sectional. It would still protrude around 6"...which is acceptable I guess.
Alternatively, take a look at the angled wall. The cleanout is located there, at around 30" high, so I have the same option there of building a low alcove for the sub as well. Again, I'm not sure how this would affect acoustics (and would it send vibrations through the walls), being stuffed into an alcove.
Now to be honest, I have a guy at my house right now doing the mudding. Both these options above means I take a step back to frame out the opening, drywall, and then mud. If having the sub upfront in the left corner would be almost as good as these solutions, I may consider it. Of course, I'm looking to you to give me the best solution, and then I may need to work back from that.