Some devices have power supplies that can be set for 120/240 and some (usually smaller) devices have power supplies that can work with a wide range of input voltages. For instance, the adapter for this notebook PC works 50-60 Hz, 100-240 volts AC.

So you need to consult you owners manual on what voltages are acceptable for the device and how to configure it.

Converting a recepticle over to 240 isn't a big thing usually, but as Gonk says, there are different blade configurations for the recpticles and plugs for different voltage and current ratings.

After all that, we listen to music, so in reality it's probably a waste of time for anyone using their amps for listening rather than as a heater.

But it is an odd spec, for sure.