Can't say yet if this is the best tweak ever, but I found a great way to trap the bass & reduce the high frequency echo of the room. I placed a 36" in length OPTP physical therapy roller in the most offensive corner, and it's amazing how much more of the music (and less of the room) I'm hearing. I can't say if it's the 'correct' thickness of a trap, but it works really well. I'm going to get some of the semi-circle types and cut them in half, so they will fit into the corners. They are less than $20 apiece, depending upon the size you buy.
Howard