hos is SYNC setup? With a single digital unit connected to your computer, the H8000FW, the clock source woiuld be better if set to internal. Working at 44.1 or 48 KHz is recommended.
A second very important detail is ROUTING. You need to select a proper routing for FireWire connection. If you access the Routing area in the H8000Fw, select routing #14 FIREWIRE 1 A_B to have the 2 dsps in series, processing 8 audio channels OR routing #21 FIREWIRE 8 Track AB in which the 2 dsps are in parallel and each one process 4 channels.
Yes, you should be able to send/return audio from/to Logic to/from the H8000FW. Refer to Logic manual for this.
We haven't tested the H8000FW with hardware digital mixers, so we can't advice. But very recently there are some postings from people who have had success working with the Mackie Onyx mixer thru firewire. Here's the thread:
We don't know about other attempts with similar technology products at the moment.