Although there's no hiss coming from the speakers when I turn the volume up to max, as soon as I plug the mic in (switched off) there's slight hiss when you turn the volume up to max... and that's with the mic trim set at minimum.

Naturally, as Dave mentioned, it disappears when you switch the noise gate on.

It's not just hiss though, turn the mic trim past 12 o'clock and you get a nice little buzzing sound accompanying the hiss too.

Cheers,
Rich