[nst7]

The C600 has assignable volume/monitor control for up to 5.1, so I assume that's what you're using, and would like an interface with similar capability.

I would also recommend Focusrite. The Scarlett 18i20 seems like a good fit. It's a USB version of the Saffire Pro 40, which is firewire. But the Scarlett is newer, and people seem to like it.

HDMI multichannel audio out from the Mac works very well, and would be an option if you wanted to use a home theater system for monitoring. But I'm guessing you already have proper studio monitors, so an interface would be the way to go.