Default stereo output bus requires ports 1/2 ?

Hello, so I noticed something interesting about Cubase a few days ago: It would appear that the default stereo output bus must be assigned to ports 1/2 of the audio interface in order to produce any sound?

I’m using the Prism Orpheus as an audio interface, and Cubase 5.5 running on Mac OSX 10.6.8. I have a headphone amp hooked up to outputs 1/2, and monitors hooked up to outputs 3/4.

I create a new Cubase project, record an audio track. Waveform looks good and all.

In Cubase’s VST Connections, if the stereo output bus is left on Orpheus 1/2, and I play back the track, all is well: The sound comes through the headphones connected to the headphone amp. However, now in VST Connections if I change the stereo output bus to go to Orpheus 3/4 and play back the track, I get nothing through the monitors. The output meters in Cubase are all showing activity, but the meters in the Orpheus Control Panel application don’t show any activity.

I can verify that other applications (e.g. iTunes) can make use of outputs 3/4 by opening the Mac OS “Audio/MIDI Setup” utility, clicking the “Configure Speakers” button, and setting the speakers to Line 3/4. But this seems to have no effect on Cubase (nor would I really expect it to).

Now, If I leave the stereo output bus in Cubase routed to Orpheus 1/2, and add a NEW output bus that’s routed to Orpheus 3/4, and then set the audio track’s output to this new bus, that appears to work.

So the question is: Is this a known phenomenon of Cubase? Can anyone else verify this behavior with other audio interfaces?

Thanks a lot!