Mmm. You could potentially use nothing but the new MacPro built-in audio over HDMI (to an outboard HDMI decoder of course, capable of 6 channel PCM, you may already have a surround sound processor/preamp capable/suitable for this?). We'll have to see how well that works in practice.
I am assuming you do not need/want 5.1 monitor control or anything else funky? Or have that in an outboard controller now.
If you are tied to the Mac, them maybe look at the Apogee Quartet
http://www.apogeedigital.com/products/quartet.php. But you are paying for preamps and inputs you don't need. That recommendation is partly prompted by how nice a lot of Apogee stuff sounds and them doing a good job (most of the time) with driver support etc. No Windows compatibility with Apogee, in case that is ever an issue.
There are lots and lots of low-price USB interfaces that have 6 or more outputs. e.g.
http://us.focusrite.com/usb-audio-in...s/scarlett-8i6. I give Focusrite kudos for good customer support, arguably better online/forum support than when than buying an Avid interface.
All the Firewire interfaces should work on the Mac Pro will work with Apple's low-cost Thunderbolt to Firewire adapter.
BTW one reservation with the Apollo is its console app does not directly support 5.1, but that would not be relevant since you won't be using it. It seems far too much if you are not even tracking.