HI: Scott
The Yamaha MS400 powered speakers don't have
any hiss from the horn or any place else for
that matter, unless you got the mixer volume
turned way up & pushing them hard, then if
your not playing there is a little hiss, but
not to bad.
I don't know if the other powered speakers
work the same as the MS400's or not, but with
the MS400's I turn the volume all the way up
on the speakers & regulate the volume with
the mixer.
I know the FBT powered speakers that I had
didn't work that way if you turned the volume
way up on them they had alot of hiss.
Enjoying the 9000 PRO
Denny