Looks like this is indeed the case.
SPDIF is not active on the Firebox unless it's set as the clock source in the Firebox control panel. This activates it. It appears that the mic pres pass through to the outs 1 and 2. I didn't use the line inputs the other night so I'm not sure what they do, but my guess is they go to outputs 3 and 4. SPDIF in routes automatically to SPDIF out.
I didn't look too closely at the routing possibilities in the Firebox software, as I was busy testing out a bunch of other stuff as well. I'll give it another go one night this week though, and report back.
FWIW - 4 channels ROCK FUCKING SOLID on my laptop, at 24/96.
Dirk