Hmmm...

Out of curiosity, do you have a a subwoofer connected to your system? Cuz its really perplexed that you're getting boomy bass from the X-LS, esp when your speakers are 5' from the front wall..
If you
do have a Sub, it may be playing up too high for the XLS, and adjusting the crossover down lower may help, or adjusting the phase if your sub allows for it. (If its just a 0/180 switch, flip it to the opposite setting, if it sounds like the bass totally vanished, the sub is playing up too high)
Does the boominess change if you move around the room?
What are the basic dimensions of your room?
Does your system have a "bass boost" or EQ feature? Have you tried adjusting those?