I'm playing back 16 audio tracks (combinations of stereo and mono), two MIDI tracks, 1 VI track. Two plugins (all my gear is external so I don't use much in the way of plugins anyway), plus I am mixer concentric (Soundcraft LX7ii-24).
At the same I was playing this cacophony back, I was recording seven tracks of audio (3 stereo and 1 mono).
Using an external drive on the same FW bus as one of the audio interfaces.
Darned if I could get it to peak, snap, crackle or pop. But I did notice that when I opened a project, I could push DP in the Activity Monitor to 120%. That usually lit up the peak indicator in DP's audio performance window. I didn't run the Activity Monitor at all when I was recording.
So you guys using networks, no network, or a single network (router or switch?), dual networks like I am (ethernet and airport), external drives or internal. Maybe if we pool some of this more specific stuff we might all benefit from it.
Boy I hate debugging someone elses software, it's bad enough when I have to debug my own...

Mel