Thank you all for the warm welcome and the great comments and suggestions! I will certainly experiment with placement and treatments. A full wavelength at 75hz is 22 feet, which is just about exactly the width of the room (with parallel walls). So this could be the problem.
Mike - you got it right on the nose with the audible thinness. This room has always had a problem with a broad dip centered around 200hz. Speakers that sound fine in other rooms usually sound thin in this room. The room is quite "hard", with concrete floors and parallel walls and ceiling made of drywall. I was thinking that carpeting or rugs might help.
What was/is confusing me is that I've not had problems with a 75hz peak in the room before. For instance, here are measurements of 5 different speakers, taken in this same room and under similar conditions.
Also, when I measure the X-Omni in the exact some location and conditions, I don't get the same peak in the 75hz range. I do get the 180hz dip, quite badly.
I will have some fun moving things around to try to improve this room and do some more measurements. This X-series is very cool and very fun!