Annoying Woofer Hum

Hi, I've recently purchased some 5.1 speakers from America and have needed to buy a separate 500W step-down transformer to connect my sub to a power outlet. This is all good and well but when I plug the sub into the transformer, the sub regurgitates an audible hum which while isn't unbearable, is quite annoying. The transformer itself produces a very small amount of noise and is brand-new. So far I haven’t bought a receiver but I'm getting this problem without anything else plugged in. The woofer generates this hum even when its "power mode" is in the off position, although I'm guessing that has to do with the receiver.

Any help is gladly appreciated, although please don't comment on why I have a system from America, the idea is looking more and more stupid by the minute