Be Safe and Power Down
Always remember to turn off any component in your audio system before connecting or disconnecting cables or wiring. With speaker wire you run the risk of creating a "short circuit" if the same (bare)wire touches both binding posts. This is surprisingly easy to do as the binding posts are located in very close proximity and you are often working at uncomfortable angles while reaching around the receiver itself to the rear panel. Modern amps usually have protection that will cause a shutdown but no permanent damage. But why take any chances? With cabling you can still create a "short" just at a lower voltage level. I've damaged subwoofer amplifiers in the past simply by forgetting to turn the subwoofer off before removing the cable from it. I have also heard of damage to other components as well(DVD players and Receivers) although haven't experienced that for myself. Again, most of the time removing a cable without shutting down the component will not cause any problems. But unless danger is your middle name....why take any chances?
Power Sound Audio