If it's mainly for ambient music, you could get an old AVR (you can find them cheap nowadays and some very good brands), then play around with the different surround sound modes. I did that in my workshop. Sounds fine for background music. You don't need to hook up the center speaker unless you have a place for it, same for a sub. I have it with 2 sets of mis-matched speakers, I tried all the surround-sound modes (Movie, Stadium, Hall, etc. that most AVR's have) and it sounds quite good.
Yeah, might be a bit odd of a solution, but it works very well, and would definitely sound better then mono or an A+B setup. Plus you don't have to worry about wiring it weird/wrong or the Ohms. Just an idea that works for me.