It sounds like maybe his speaker cables weren't connected very well and he inadvertently fixed it. I don't think it had anything to do with the phase. Your problem also does not sound like a phase problem... things that are having phase problems don't just sound less than great, they sound totally weird. If you're not sure what's + and - on your outputs/inputs you can check to see which sounds better, but assuming everything on your gear is labeled, and labeled correctly, you want it the way it is... the same color should be on + for each speaker and likewise for -.
One thing that's helped me troubleshoot before was listening up close to each part of the speaker (ie. each cone, the tweeter, etc) and comparing the two. Often times I'll find that on one of the speakers the mids are a lot lower, and sure enough will find a loosely connected speaker cable on that speaker!