without a question multichannel master audio would be the way to go for a readily mixed 6 channel source...
but:
the multichannel audio mode is a bit wired to setup and you don't see the config directly in the audio mixer
- 5.1 panner is visual logical structured - you see if a channel called left isn't panned to the left
- multichannel master has all this stereo mixing knobs that have to set right and the setup is confusing:
if you want to: get 6 mono channels
channel - assignment
1 - L
2 - R
3 - C
4 - Lfe
5 - Ls
6 - Rs
you have to setup:
channel - assignment
1+2 pan 100% left - L
1+2 pan 100% right - R
3+4 pan 100% left - Ls
3+4 pan 100% right - Rs
5+6 pan 100% left - C
5+6 pan 100% right - Lfe
I think this has it's roots in the stereo pairs of tape decks - but it's not called "legacy tape deck mode" ;-)
so I would prefer a more logically structured setup...