How to stop the boom in the low frequencies

I have a pair of Gershman X-1+Sw1 Subwoofers and have them paired up with a DNA 225 Amp. The Preamp is an old Sumo Athena. Although I am satisfied with the over tonality of the system, what really bothers me are the really quick bass lines that tend to hang and make a slapping sound against the walls. I have tried to stick the speakers out into the room, without improvement. What will tame the bass?
Some tweeks for the amp or the preamp? Cones etc...?
Some sound deadening panels behind the speakers?
or
Some sort of device that simply lowers the registers in the low end?

I need specifics of brands and results,also where purchase them.

Thanks