I wouldn't clear the speakers until you tried another pair. I'm thinking there could be slight differences in the way the sound is processed in each of the three modes. Maybe the DSP wasn't programmed right or maybe the wiring is slightly faulty. Whichever is the case it seems like the speakers are pushing the gain up on background noise and it is possible there's some sort of feedback loop going on.

Apart from that, I wouldn't expect £70 speakers to pump out white noise. Even my old Genius speakers with their relatively high noise floor can soon overwhelm the tiny amount of hiss with the sounds you actually want out.