What if you used 240V UK supply to power 120V US Bose speakers?

Don't do it. Period. You will overheat the wires , blow the board and start a fire within seconds. I assume you're talking about the pre amp and the amplifier. Most sound systems run their own power supplies to the speakers. Do this instead. Check the Bose speakers and see if they have a voltage differential switch, some do especially if you're in Europe or the UK. If they do not and you are in the UK you will need to get a transformer to step down the voltage to 120V 60 hertz. Be sure to match the amperage of your system to the transformer you get. My personal advice is not to use the speakers at all as the price of the transformer and the hum that it produces compromises the benefit you might derive from the sound system.