I don't have 5.1 on my Mac [I have a dedicated PC I use for movies], so this is a bit of a guess...
Apps > Utilities > Audio Midi Setup. Should be able to adjust individual channel volumes in there.
This is how my stereo output looks. Also check in 'Configure Speakers...' that it's set up for 5.1
Edit - I just realised you said only 2 speakers... not sure how that's going to work, unless you're pre-mixing at the Mac as 5.1 then muxing back to stereo in your amp.