Audio from one computer to the other

[Cubehead 666]

I'm running DP 6 here on my main setup. I'm also running DP 4.5 on my older iMac, because Spectrasonics has no update to run Atmosphere on Intel Macs. I have an M-Audio Duo here and a Quattro on the iMac, with outputs running from the Quattro to the Duo. When I run my synthesizer, the Duo receives nothing but harsh, overamplified noise. So much, really, that I thought I might have blown my speakers. They're fine. I added a limiter and lowered the volumes on both computers to be safe.

If I run Atmosphere straight thru the iMac's built-in audio, it sounds fine. But if I run it from the Quattro to the Duo, I get the harsh noise. I've twiddled with the line level in/out buttons on both interfaces. Nothing. I guess I should note that I have both ins on the Duo occupied with instrument cables from the Quattro, as well as a mic cable in one channel on the Duo. I left it in, because getting that thing out is a pain in the ass, and the manual says the instrument cable supersedes the mic cable.

Any takers on this one?