OOoo I have all kinds of ins and outs!
I have a USB2 Multichannel analog out to 5.1 Surround for use with STP and also set up to monitor separate tracks in FCP on 5.1 setups also the optical SPDIF connected to the same Surround for DVD playback.
I have a Stereo setup connected from the Mac and also from the Blackmagic Multibridge.
My suggestion would be invest in a decent pair of Headphones and Audio Monitors (Speakers) regardless of what output you monitor on them whether directly from the Mac or via an interface or video card.
Something like these which I recently installed in an Edit Suite for a client.
MACKIE HR824 MK2 ACTIVE STUDIO MONITOR (PAIR)
[
www.mackie.com]
Sennheiser HD 280 Pro Headphones
[
www.sennheiser.co.uk]
And don't skimp on the cabling either! Get pro quality audio cables.
If you ever get a ground loop hum (it does happen quite frequently even on pro setups) then there are a variety of ways this can be rectified and a simple google search will provide a wealth of solutions.
For instant answers to more than one hundred common FCP questions, check out the LAFCPUG FAQ Wiki here : [www.lafcpug.org]