Davide,
I use a FireFace 800 and an H8000FW. There are many ways to connect them; I'll tell you what I do, which works well for me.
I use Fireface as the primary interface to the outside world and I use the H8000 as an effects processor. This means that although the H8000 has great I/O abilities I don't plug anything into it except the Fireface. I connect the FireFace to my system with the FireWire cable.
I use ADAT to hook the H8000 up to the FireFace. This means two cables: one from FireFace ADAT 1 OUT to H8000 ADAT IN, and one from FireFace ADAT 1 IN to H8000 ADAT OUT. These are fiber optic cables and you have to find ones that seat properly in the H8000, which has a socket configuration that many cables don't work well with.
I use the FireFace as the master digital clock in the system with the H8000 set to slave. I use a pair of clock cables to connect the FireFace clock out to the H8000 clock in and vice-versa.
I use the FireFace Matrix to route eight channels to the FireFace ADAT OUT. This sends digital audio to the H8000. I pick up eight channels of return from the H8000 in the same way.
Since your FireFace sends all 24 channels up to your PC through the FireWire link, you have access to the H8000 return audio the same as any other FireFace input. Depending on how you set up your audio software and the FireFace matrix, you can use the H8000 as a preprocessor (effecting the audio before your software sees it) or as an effects loop (send and return from your virtual mixer console). I use both depending on the project.
One of the great things about using the FireFace and H8000 in this manner is that it is easy to record both a dry (unprocessed) input and wet (effected) input at the same time, to spearate tracks. That way if you are unhappy with the H8000 settings during a recording session you still have the original source material and you can just send it through again (guitarists call this re-amping).
