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?

bryanhod
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.