OK - I just did a test. I have a 4-channel (Quadraphonic) aggregate device defined in Audio MIDI Setup using the green and black plugs. I selected this as my default output in System Preferences. It is true that System Preferences does not show any volume slider for the aggregate device. I'm not sure why, but that hardly matters, since the volume is usually able to be controlled from the application playing the audio.

I then opened an MP3 in Quicktime Player. I then went to Windows > Show Movie Properties. In the resultant window, I selected Sound Track, and then Audio Settings, to get to the list of channel assignments.

I then plugged my earphones into the green plug, and played the file. It sounded fine. I then switched right and left channels to right surround and left surround respectively. Now I heard silence. I then plugged my phones into the black plug, and lo-and-behold, the sound was playing there, as expected.

My aggregate device works perfectly, and I now have the ability to play quadrophonic mixes (Dark Side of the Moon, here I come...). I fully expect that if I get the headphone output working, I'll have a full six channels, and can assign them properly for 5.1 output.

I'm not sure what you're doing, exactly, but I'm having success.

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