I was wondering how much does having a second, passive subwoofer in the back add? I generally listen to music and 5.1 movies. I got this idea reading through my receiver manual and noticed it had setup instructions for adding in surround back speakers along with an optional setup for a second sub. It says to wire the second sub from an output on the L surround back and the opposite output of the R surround back. It also labels it as "Subwoofer (Passive)". Any thoughts on it's purpose, performance, etc?