Ok, so it sounds like this is an issue different from what I experienced. Since they are hissing even without a source attached that pretty much just leaves the speakers themselves or environmental interference. If it's at all possible I'd try setting them up in a different location, if you're curious.
I also had to use an isolated power supply in my setup to avoid hum, but that doesn't seem to be the pervasive issue for you right now. I'm hopeful that Laine will take care of you, given the amount of business that's been thrown his way lately by this site.