Not sure why you would want to use the inferior Mac audio for monitoring when you have an audio interface that has better (more accurate) sound.
The System Audio Prefs have -nothing- to do with Logic.
You can upgrade to Logic 9
Create an aggregate device:
Ah, good point. I decided to just hook the monitors up to the interface. I had tried listening to playback through headphones plugged into it, but the sound was sub-par. But with my flat response monitors delivering the sound from the interface, the result is faithful reproduction of the recorded audio.
Good to know about aggregate device as well -- that may come in handy for other situations.
Thanks for the info.