Sounds like room acoustics to me. A room of that size can be a great audio room. Your equipment definitely sounds up to the task.
Before doing anything permanent. Start by simulating room treatments. Get all the couch cushions and pile up behind the speakers. Find a way to lay temporary carpets... if necessary, pull out multiple layers of towels and all the bedding and cover the floor. You want to deaden behind the speakers, and reflection points on the sidewalls and the floors.
I just finished helping a friend turn an extra room about the size of yours into and audio room. It sounds fantastic. We started with couch cushions, pillows and carpets and then over the last year he purchased new and used ASC tube traps and panels. The room sounds fantastic with Wilson Watt Puppies. You can turn it up loud and it sounds great, dynamic with deep punchy bass.
Ultimately we chose the narrow wall to place the speakers.