It's worth noting, too, that Ohms Law alone is not an accurate way of determining the output power of an amlifier into a given load.

Other factors, such as the actual voltage an amplifier is capable of producing at a given impedance, can seriously affect the power.

As an example, it is possible that an amplifier channel rated at 500 watts into 4 ohms may produce as much as 350 watts into 8 ohms, and not the 250 watts that Ohms Law would suggest. However, two identical 8 ohm loudspeakers would share the 500 watts at 4 ohms and demand 250 watts each.

When in doubt, consult the manufacturer's specification! If you don't have it, then be very cautious!

Being silly about impedances on a valve amplifier can result in burned out valve bases, too.