Tweater hiss

If you unplug your preamp from the amp, do you still have the hiss? Does the hiss change in volume when you adjust the volume of the preamp?

Some low level hiss is normal when the electronics are powered up. It should not be a loud hiss.

I checked it out last night and when the volume is turned up, the hiss increases in volume. If the preamp is unplugged the hiss is still there.

That makes little sense to me.

If the his varies with the volume control, it comes from the preamp or the source.

If the hiss comes from the preamp or source, it should disappear when the preamp is unplugged from the amp. Are you sure it is completely disconnected from the system and that there are shorting plugs in amp's inputs?

The fact that you isolated your amp from the rest of the system would narrow it down to a couple of possible explanations that I would check first.
1. The noise is typical of the unit (ask Bryston)
2. Try an AC line conditioner. (garbage in - garbage out)
3. If not 1 or 2, I would be asking someone smarter than myself.