That was exactly the same fault I had with my girlfriends Ref.405, but in one of the smaller channels.
Does yours have a turn-off pop on the wooferchannel too?
It's just a turn-off pop, that's what made me curious.
What I read so far, that's common to many older Soundstream amps.
I posted a threat here, but never had the time to take it out of my girlfriends booth and look further.
