In my experience, the noise on a Zed 14 should actually be very good--nearer -85dB on the Audition meters than the -51 you mention. The only time I've heard this sort of hiss from an A&H mixer, the fault turned out to be a bad capacitor in the power supply and A&H were very good about fixing it.
However, what worries me about suggesting this is that you say the headphone output is clean (though it can be hard to tell).
Can you plug some decent speakers into the L/R output of the mixer and listen carefully--probably while trying connecting and disconnecting different sources to see if one is bad.
Suitespot's advice about checking your gain structure is also good...though even turned right up, the mic pre amps sound darn quiet to me.
As always, it's also worth updating/re-installing the USB drivers that come with the mixer.
Thanks so much for your input. I seemed to have narrowed the problem down to the default format in the microphone properties tab in Windows 7. Made a simple settings change (through experimentation, of course) and the hiss is totally wiped off the map. After walking away in total disgust yesterday, I came back this morning with a cool head and addressed the issue.
As mentioned, I'm still very new to CS5.5 and still have plenty to learn. I'm sure they'll be a few more bumps in the road as I go along.
Ryclark got there before me but I agree something sounds wrong. The Microphone setting in Windows should have nothing to do with the output from the Zed.
If you haven't already done so, go to Edit/Preferences/'Audio Hardware. You should be able to set this to an ASIO driver and select the output of the Zed 14 as you sound device. You may also want to go to Audio Channel Mapping and tell Audition what track to record your L/R outputs to.
Finally, you have to adjust the controls on the Zed itself to decide what to output to the USB...probably just the main post fade L/R outputs unless you want something else, but A&H gives you several choices.
Hope this helps,