emptybe
The d8b... Is it OK with X2 & X3?
Yes.
emptybe
So I'd need a piece of additional software for it to 'talk' to Sonar?
Yes, either software or hardware/firmware. You need a translator/emulator to convert from d8b native RS232 serial messages (formerly to/from the d8b CPU) into Mackie MCU MIDI messages and back again. The emulator conceptually sits between the d8b console and your DAW. The D8Bridge solution does this emulation in software running inside the DAW and makes use of virtual MIDI ports. The ProBox accomplishes the same thing in an external box that physically sits between your DAW and the d8b control surface.
The D8Bridge v1.1 software development effort has been abandoned by its author but the software is out these and it works. The ProBox solution is still actively supported by its developer.
emptybe
But the audio I/O is integrated with the control surface functions like my FW-1884?
For D8Bridge, no; you'd need an external audio management system / monitor controller. For the ProBox, all of the audio functions you would expect are executed by the ProBox and made available on the d8b control surface.
Unlike the Behringer, Mackie MCU Pro, and some other turnkey MIDI controller solutions, you need to be a bit of a systems integrator to get where you want to be. It's not bad, but it just takes a little more than plugging it in and turning it on (although the ProBox is real close to that level of simplicity).