Ok have you tried adding an aggregate device for the 6 channel setup and forcing it back to 2 channel? it might work/worth a shot. Open up the Audio Midi setup in utilities. Click up in the top bar on audio and Open Aggregate Device Manager. Make a new Aggregate for 4 channel and one for 6 channel. Then select the properties of each Aggregate Device and Change the Audio Output for each Stream Back to 2 channel. I don't currently have Garage band installed to test myself.