I have 3 Defenders in my systems.
I use the Defender when I'm not using some other type of power conditioner...e.g., when I have a sub on an duplex, I put the Defender in one of the outlets. I'm not sure it makes much difference in sound quality TBH...it might...but I haven't really tested it...I mostly use it for surge protection.
I also use a Defender with my TV and I can say that using it there results in a bit sharper picture on the plasma set. But again, I use it mostly for the surge protection.