From your description, it sounds as though the input level that you are supplying is way too high, thus driving the little amp into clipping. Try turning down the volume on the device suppplying the input. This might fix the problem. If you can't get a clean signal even from a low input, maybe you've just got a faulty one. Those good reviews can't ALL be lying!
The real point of an amplifier like this is not to boost volume levels beyond what your device normally supplies, but to allow four users each to have her/his own choice of volume level from a single source. If you just want huge volume into a single pair of cans, this is not your device of choice.