Bass traps are acoustical treatment devices used in room tuning that absorb the long bass wave energy that often get energized in the corners of a home theater room. These in-wall or floor-standing devices range in price from a few hundred dollars to thousands of dollars. They help to clear up that "clogged-up" or "muddy" bass sound, thereby making a room sound tighter, louder and more in control on the low end.
Check out room acoustic and bass trap products from Acoustic Innovations, Acoustic Sciences, Acoustics First, and Core Audio Designs.
Also check out subwoofers, AV Reveivers, and AV Preamps that include digital EQs that can do a similar job electronically what bass traps are doing physically.