I think this is a very common thing. I can't think of a rig I've assembled in which this "hiss" didn't exist at some level. The degree varies from preamp to preamp. It was a big problem with an Exposure preamp I had, which was a really great preamp in spite of it. I was told that a common method to reduce it is to place caps on un-used RCA inputs. I just decided to quit worrying about it instead.
