Meridian processors provide most of what you are looking for. G68XXV that I used own had excellent DAC, pretty good two channel section, and ability to upsample 16/44 redbook PCM. If I recall correctly Meridian processors digitize analog inputs in order to apply DSP and room correction. I don't remember if A/D can be bypassed.
However I'm still not clear why you would want the processor to "downmix" 5.1 to two channel. All DVDs contain two channel PCM audio track which is typically higher quality than DD or DTS surround track anyways. MCH SACD and DVD Audio include two channel mix also.
However I'm still not clear why you would want the processor to "downmix" 5.1 to two channel. All DVDs contain two channel PCM audio track which is typically higher quality than DD or DTS surround track anyways. MCH SACD and DVD Audio include two channel mix also.