FireWire from iMac to a Mackie Onyx 1220 needs a driver?

I am running FireWire from my iMac to a mackie ONYX 1220 mixer. I shouldn't have to install any special drivers because the Mackie 1220 uses the core audio drivers of the iMac. Yet, I get no low frequencies coming from the speakers. When I unplug the FireWire mid song and then plug it back in, I get the full, beautiful sound I'm looking for for about 1 second. Then it goes back to only the high end frequencies.

Anybody have any answer for me? I took this mixer to a Mackie service center and there's nothing wrong with it. It works fine on other computers.

Thanks, Tom