It looks like that 8 inch subwoofer might be a weak link in your system. It would be a good start to replace it with a 10 or 12 inch sealed type . Take a look at these!
http://www.jlaudio.com/home-audio-powered-subwoofers-e-sub
http://www.theabsolutesound.com/articles/jl-audio-e-sub-e110-subwoofer/
http://www.jlaudio.com/home-audio-powered-subwoofers-e-sub
http://www.theabsolutesound.com/articles/jl-audio-e-sub-e110-subwoofer/