I have one and we use it on both a DV 400 iMac and a Pismo 500 with just FW power.
When you plug in the device via FW do the LEDs light up? I get a green one in the middel and the red one on the right. Also, the control strip module should change.When the device is plugged in the speaker control becomes active. When I unplug it the speaker part of the ProTV control panel disappears.
I assume you have already tried some of these but...
Reinstall the ProTV software.
Did you install anything else related to FW? A hard drive? CDRW? Toast? Maybe it is interfering?
Can you take it to a friend's and check it on their iMac?
Reinstall Mac OS, perhaps something related to FW is corrupted. I've had no trouble on two computers running the US Mac OS. However, I tried to install it and run it on Japanese Mac OS and it absolutely hosed the system. I tried it twice and each time I used it I was forced to reinstall the OS. Although I have never had trouble on the two computers running US Mac OS it makes me suspicious that something could go wrong.