Are you going to record in FCP? i haven't tried audio only with the ioHD yet but sometimes FCP likes to see a video signal even if you aren't recording it. ANy video signal will do such as a DVD player or camera. As long as you a re feeding video to it, it won't matter what it is. Now, that might not be true anymore but it's the way it used to be.
Best to use the ioHD with Soundtrack for audio only.
Then again the latency is quite big while monitoring.
It works but I haven't really put it through its paces yet.
I needed to record a quick VO but the delay drove me mad so I switch to the internal audio in/out of my powerbook for the time being.
So yes it works audio only, but you may have to figure out a good workflow first.
I finally did get it to work as a stand alone firewire interface with Logic 8. It took some manipulation but I got 8 channels of AES working. Now I will try the 4 channels of line inputs to hear the quality of the converters.