Or, if you want to make the best use out of your reference speakers, get an audio interface (USB or FireWire, or even a PCI card). Look for one with balanced XLR or TRS (phone plug) line outs. Even the cheaper ones should easily outperform the Mac's built-in line-out, which - let's face it - wasn't designed for high-end audio work.
[Edited to add: Oh, now I see you want to run a digital out to the mixer. I guess that's a little different, and with good DACs in your mixer I guess you should be fine.]
- Martin