This is due to the design of your A&H Zed14.
If you take a look at the manual for your device [page 22]:
You will see that the unit, when connected to your iMac uses the iMac's core audio.
**Please make sure your OSX version is compatible as well. This unit was manufacutered prior to Win8, Win10, OSX 10.8+ and there have been some changes to how audio is handled in these more modern OS versions.
Basically, the ZED allows 2in/2out and works on the USB CODEC; as described in the manual and per it's design. The ZED does not allow for setting up of all 14 channels in and DAW's I/O.