I'll chime in here primarily in agreement with what has been said. The response looks very good, and the bump below 20hz can only help IMO.
What would make a big audible difference would the the addition of good well placed bass traps. What you're probably hearing is the ringing as shown on the waterfall graph. You'd quite likely be able to bring up the null some at 97hz, and knock down most of the ringing you're getting down into the mid 50's or so. The result would be smoother integration with your mains and a more detailed sound. The ringing below 40hz is pretty going to stay there though without more dramatic measures.
Realizing that extensive room treatments are not really an option in this case, you may be pretty much stuck with what you have. And not too shabby though. Really.
Tim
