Tweater hiss

I have B & W 802 Nautalis speakers, Bryston 4B ST amp, and BP-25 preamp. I hear a hiss coming from my tweater when no music is playing. I assume it is occuring during the playing as well. I just had the amp serviced by Bryston and was wondering if anyone had any ideas on what is going on?

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.
Kr4 that is exactly what I thought. I was expecting that once I disconected the preamp, the hiss would go away since it increased when I turnded the volume up. I have a Audio Matrix power conditioner coming on Friday so maybe this will help. I am also having a dedicated line put in soon as well. I will contact Bryston about it as well.
How much hiss is there? Can you hear it when you are more than a foot from the tweeter? If not, it's probably OK. Otherwise, see what Bryston says.
I listend again last night and the hiss can be heard about one foot away but not more than that. The volume increase is minimal when the volume is increased on the preamp. My guess is it coming from the amp. I will put on the power conditioner and see if that helps. Thanks for the information and help.
I installed the Audio Magic power conditioner and this reduced the hiss. Also greatly improved the sonic clearity of the music. I will wait to hear from Bryston on the amount of hiss that is normal. Thanks for your help.