John,
The "AC" that comes out of your amp is not necessarily 120Hz; it's audio band signal so it ranges from 20 Hz to 20 kHz. Maybe your thinking about wall household input frequency???
anyway, Your amp, any audio amp, in general basically modulates a DC power supply with the input waveform, to produce the amplified AC output. One could easily reverse both left and right speaker cable polarity (eg swap red/blk) and not notice a bit of difference (forget about active subs and the possible issues with phase there).
(aside-If all you heard was a 120 hz signal your power supply filter caps are probably out to lunch.)