Correct AC line polarity can enhance most equipment's sound. To perform this, you'll need a multimeter and a set of what is called "cheater plugs". You've seen them, those two to three prong adapters for use with drills, electric saws, etc.. Usually they are orange or grey in color. They can be bought at Target, Home Depot, and almost any hardware store. One male prong is bigger than the other. The bigger one must be filed or ground to the same size as the smaller one.
Disconnect all cables from the piece of equipment you're going to measure first. Make sure the power switch is off. Please note that most amps need to have speakers hooked to them when they are on. So, on amps leave the speakers attached. Place the cheater plug on the AC cord of the unit. Attach the negative lead of the voltmeter to a good ground. I like to use the screw on the AC receptacle's wall plate. Connect the positive lead to a place on the unit's chassis. I usually just touch it to a screw. Set the voltmeter to the AC Volts position. Now plug in the unit and turn it on. Wait about a minute, then note the meter reading. Shut the unit off, reverse the AC plug in the outlet. Turn unit on, wait a minute, note reading again. The lower reading is the correct plug orientation. Do this for the remaining audio equipment pieces.
You can take this one step further. Listen to the system with the plugs in the correct position. Then listen to the system with ALL the plugs reversed 180 degrees. One way should sound better than the other.