Hmm. Hiss normally comes from early on in the amplifying chain i.e the recording or playback pre-amps but I wonder if in this case it's from the headphone amp. The headphone output is meant for just that, not really for connecting to speakers, powered or not. It may be that you're having to turn the gain way up high and therefore hearing the hiss. I'd recommend trying to connect to the line outs.

Not sure I can suggest much more.

Where are you? Long shot but if you're in the East Midlands I could take a look/listen at it.

Mitch