I run two separate stereo systems. One reason is I am constantly trying out new systems to see which one I want to keep. The same for speakers. Bring them in, try them out, move them on if they don’t meet the cut. This little device is a wonder for that! Don’t have to crawl around rewiring speakers from one receiver to the other. I can have two receivers hooked up to the same CD player (using another device) and switch speakers seamlessly while it is playing. It’s a dream for comparisons - or just for deciding which system I want to play one particular CD on.There is absolutely no hum or any interference when using this. I run my A speaker outputs from my receivers to the box, then run from there two two sets of speakers. I have added a third speaker set by simply connecting that one to the B output from one of the receivers. Could have 4 sets of speakers set up to run by flicking switches if I wanted to (but can only “compare” two sets at a time) - or get another box like this and run the Bs to it!What else can I say? So very cool!