BookShelf Speakers Sound Muddy HELP


Hello,
I have a pair of Sonus Faber Concertos (not the domus). I also have a Vandersteen Sub. I notice that the bass coming from the speakers sounds a little muddy. The highest crossover setting on the Vandersteen is 80. Is there something I can do to tighten up the bass on the Concertos. Could this be a placement issue? I have the speakers on stands about 18 inches from the wall.
autodexr