Here's more to show that I don't think I need side-wall treatments.

I measured the direct distance from my speakers to my listening position and all of the major reflection distances (see spreadsheet). I then used the reflection coefficients for the materials that caused the reflections and the distance of the reflected path to calculate the reflection SPL. I then figured out the reflection delay. This is all according to the Master Handbook of Acoustics.

I used the attached chart from the book to figure out which reflected paths cause me grief. Grief = echo + imaging effects such as shifting and spreading as opposed to spaciousness.

The only areas that could be cause for alarm are the floor and the ceiling between the listening position and the speakers (shown in yellow on the spreadsheet).