Well, I'm slowly figuring out through a process of elimination what the problem isn't....
I managed to get an upgrade for the Digi items, although even this isn't fully compatible with 10.5.6 (PT7.4.2 is apparently in the testing stages; only the new PT8 is fully approved for the newest Leopard, and it's not free). But it gives me signal in both ProTools and Logic, so that's a start.
However, once I got that far, I realized that even with the M-Audio box and drivers eliminated, I'm still getting noise from the MBox. The only difference between my set-up before and the current one is the MBox is now directly connected to my Korg keyboard via 2 mono instrument cables. So it seems that there are two possibilities now: A) either the Korg is always noisy (ground problem?) or B) I'm using poor quality cables. Is SPDIF potentially a better connection?