I have never heard of swapping the plus and minus connections on a passive speaker cause any amp damage either. Like Almarg said, if the speaker is AC powered somehow, or a powered subwoofer that may tie the two plus or minus outputs from the amp, yes, that can cause damage. Regular passive, never heard of it, just as long as the left and right channels connections stay isolated from each other.
If this wide sound stage happened on its own, there may be a bad connection in a component, or one of its semiconductors somewhere. With the ground, or other connection opening somewhere, I think there is a possibility that it is starting to give the ambient information (the difference between the two channels) and acting like a matrix surround sound coming through your mains speaker.
Double check all of your speaker connections like mentioned.
If this is a vinyl system, double check your cartridge and turntable connections.
If you have any recording that you know that has an instrument, or vocal that comes *mostly* but not totally out of one channel before, try it and see if it still does. If it comes more out of both, possibly with some echo now, there is a good chance like I mentioned before about a bad connection somewhere in a component or outside (cartridge/speakers etc.) wiring. Another sign of something like this happening can be a lot of reverb/echo type of sound in a lot of your stereo music.
If this wide sound stage happened on its own, there may be a bad connection in a component, or one of its semiconductors somewhere. With the ground, or other connection opening somewhere, I think there is a possibility that it is starting to give the ambient information (the difference between the two channels) and acting like a matrix surround sound coming through your mains speaker.
Double check all of your speaker connections like mentioned.
If this is a vinyl system, double check your cartridge and turntable connections.
If you have any recording that you know that has an instrument, or vocal that comes *mostly* but not totally out of one channel before, try it and see if it still does. If it comes more out of both, possibly with some echo now, there is a good chance like I mentioned before about a bad connection somewhere in a component or outside (cartridge/speakers etc.) wiring. Another sign of something like this happening can be a lot of reverb/echo type of sound in a lot of your stereo music.