
Just wanted to offer some help.
I have been using my Zero 8 mixer for some months now, and although their is some (lots of )problems with this mixer ( I can testifie to that), obsessive hissing is not one that I am experiencing, and my last three serial # ends with 280.
I am using a TASCAM CD-X1700 dual CD player along with my PC laptop and a QSC PLX 2402 power amp.
I am coming out of the Zero mixer from the XLR +4 dBu ( Bal) output going into my power amp with 12k Balanced input with very minimum to normal hiss.
Do you hear the same hiss in the headphones or when using your computer speakers? (turn off all FX / Mixer on flat)
If you do hear that hiss from headphones or PC speakers then it's possibly could be a polarity problem or bad amp output from the Zero mixer.
If you don't hear the hiss from the headphones or another output.
I would also try using the mixer from other sources, such as another amp that I could hook up with the Zero 8, or take it to a location ( to the place of purchase, if you got it from a store)where I could hook it up using various cables and outputs or another Zero mixer if I can, to see if it's a problem with the mixers amp, cables, polarity switching, or whatever. ( As you booth know it's a process...of elimination) I am sure with the knowledge that you booth have you will get to the bottom of this issue.
Please keep us all informed while we wait very patently on Korg's announcements on the very badly needed update for the Zero 4 + 8 Mixers.

Hope that info could help you.

Peace & blessings.