I second Newbee's comment. The only things I can think of that would account for the symptoms the OP described are a +/- reversal to one (and only one) speaker, or use of a balanced interconnect that has pins 2 and 3 interchanged at one end of the cable in one (and only one) channel.
Regarding the experience Marakanetz reported, I agree with the comments by Stan and Metro, provided that the speaker is purely passive (i.e., it does not contain an amplifier), and provided that a powered sub is not connected to the outputs of the main amp.
If the main amp is connected to the inputs of another amp (either in the speakers or in a powered sub), then it is possible that a +/- reversal could cause damage. That would occur if both amplifiers have 3-prong power plugs, and both have ac safety ground in common with (i.e., connected to) their internal circuit ground. In that situation reversing + and - would short the + output of the main amp to its ground, via a path through the amp in the speaker or sub and the safety ground wiring of the two components.
Regards,
-- Al
Regarding the experience Marakanetz reported, I agree with the comments by Stan and Metro, provided that the speaker is purely passive (i.e., it does not contain an amplifier), and provided that a powered sub is not connected to the outputs of the main amp.
If the main amp is connected to the inputs of another amp (either in the speakers or in a powered sub), then it is possible that a +/- reversal could cause damage. That would occur if both amplifiers have 3-prong power plugs, and both have ac safety ground in common with (i.e., connected to) their internal circuit ground. In that situation reversing + and - would short the + output of the main amp to its ground, via a path through the amp in the speaker or sub and the safety ground wiring of the two components.
Regards,
-- Al