I don't actually own a Zero8 although I wouldn't mind owning one, but here's the fix for it without having to read a whole bunch of stuff that would otherwise scare you into thinking there was something majorly wrong with an otherwise perfectly functioning mixer. But apparently there was a hiss issue on it with a fix that kind of goes like this...

Source:  official Korg Forums : http://www.korgforums.com/forum/phpBB2/viewtopic.php?t=29285&postdays=0&postorder=asc&start=150

djcl.ear Posted: Tue Aug 23, 2011 6:31 pm

Soon after Korg acknowledged that normal operation of Zero8, produced hiss sound on its analogue output, they came up with a "solution" which basically is running all the relevant volume levels (master fader, etc) close to its upper margin below clipping.
This way the background Hiss is less perceived or masked. Obviously it still is there -as silent or lower volumed passages show- , but the actual Signal to Noise rate improves. 

Running Zero8 this way is called HOT. Because the mixer gets very hot, which adds new problems to it, mainly reducing its internal components' life. 

There are increasing users complains of failing touchpads and channels, which are a predictable effect of this HOT usage. 

The alternative is using Zero8 firewire connection and outputing the sound thru the Laptop/PC soundcard, and running it in COLD normal levels. Or alternatively using its SPDIF out plus an external DAC if your amplification system lacks this input. 
Additionally, there are some Users (from Hispasonic forum) which have opened Zero8 and added fans to improve its thermal management design. 

Hopefully many Zero8 users learn this and thus will enjoy a clean sound and long utilization of this literally wonderfull and unique mixer.