Certainly with properly authored MLP as used with DVD-A and the basis of Dolby TrueHD, the 2 channel mix is stored and the multi channel is derived from this by applying delta changes for surround info. This is a design feature in MLP that allows a track to be played even on simple 2 channel devices.

If it is the norm that discs do indeed follow this method, then having the basic 2 channel mix extracted by the source must be the most pure method. Purporting a multi channel mix would involve the source taking the 2 channel mix, adding the multi channel delta info, sending the multi channel mix to processor which then needs to fold the multi channel mix back in to the original 2 channel mix.

Certainly with MLP based discs, outputting as 2 channel would to ke seem the best method and also the simplest.

Meridian owner since 1992
Prime & PSU, Focal Elear headphones, Roon (ROCK 8Gi5 NUC), Explorer 1 & 2, F80. 200/203, various Sonos (via Roon).