I looked at the M-Audio firewire 410 when I was using SAMSC on Mac OS9 on a G4... quite a while ago now... (!) I seem to remember there were some MIDI latency issues surrounding firewire - However, I was syncing timecode... so maybe this might not be an issue for you...

I ended up using a MIDIsport 1x1 and a Super DAC2496 for the audio, connected to the optical out on the mac... - Still do when I need to send audio to a sound system with Catalyst