Does your D8B have an Apogee Sync Card? If you intend to hook the D8B into the Fireface and use the ADAT lightpipe connectors, you're going to need the Word Clock connected between the two units. Either could be the master. If there is no Apogee Sync card, it is going to make digital I/O between the D8B>RME>Computer running Ableton, problematic.
You don't tell what kind of Fireface. 400? 800? Big difference on how many channels you can pass back and forth, digitally. You can use the analog I/O between the units with nothing more than the proper cables. Did your D8B come with a MIDI connector adapter on the CPU? You're going to need one of those to get the D8B to follow MIDI Time Code from the Computer>RME>D8B chain and to send MMC back to the D8B>RME>Computer chain.
Does the D8B fire up? If so, what software version did it come with? You're going to want V5.1. There is a thread here on the subject of upgrading from v3 to 5.1. Mackie stopped giving out codes and Munk finally cracked it, so it is possible to upgrade without Mackie's complete lack of help. Read this thread:
viewtopic.php?f=2&t=928&start=410Go to page 41 and look for Munkustrap's post.
Can you use it as a big controller for Ableton? Yes. By itself, the D8B is capable of mapping 128 MIDI controls either in or out. If you just want it to be a controller only, read the ProBox forum:
viewforum.php?f=7Munkustrap can give you help there.