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