Just to prove mixing subwoofers does work (at least in theory), haven't spend that much time audibly comparing them.. basically just doing the graphs. Here is a graph of the SVS 20-39 PC Plus in stock tune, SS to 20hz, SVS xover disabled. BM setting to 80hz. In pink. In blue is both subwoofers switched on.. http://nathansilly.012webpages.com/Final.JPG Oh yeah since the xover is set to 80hz, should 80hz> drop-off quickly (like the pink line) or should it be flat (like the blue line)?