How can i get a RSNAV to play Audio through my own speakers/amp ?

[Dark Session]

So i have not bought anything yet. But i want to keep my B&O System in my RS5 8T 2010 (pre facelifft). At least the AMP/DSP/Center speaker ... just to avoid errors in the MMI. But i want to replace the 3 Way Speakers on the front doors, the two way system in the back and the sub. I don't want to use the oem wires, so my plan is to wire new cables from all new speakers to a dedicated 8 Channel AMP in the trunk together with the Subwoofer.

Now my question is, how do i get the media signal out of my (future) RSNAV S4 ? Which Sound output can be used to let it play into the amp with best quality?

Edit : Even easier and cheaper.

DSP+AMP (all in one). New Speakers, new sub. Keep the original wiring and just use the cables that go into the quadlock on the B&O AMP/DSP. I connect the cables to the new DSP/AMP and run a optical connection to the RSNAV.

Done. Everything i loose is Volume/Title controls on the Wheel and center console. But that should not be too bad because the RSNAV has a touchscreen.