Can you help me select a pre/pro?

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.

In regards to 2 channel downmix for DD/multi, I presume most/all(?) processors do this if you configure for 2.1 channels anyway. I can't remember having an AV processor that didnt' have the ability to configure for 2 channels plus subwoofer, in which case all information is downmixed to 2 channels, regardless. I could of course be mistaken.
I've had a lot of processors and receivers over the years, and can't think of anytime this aspect wasn't fully handled.

As Skippybo has said, every pre/pro I know of will do a downmix from multichannels to 2.0 or 2.1 but you would be best off using the dedicated 2 channel tracks contained on almost every disc.

Kal

It depends on the BluRay authoring. You should be able to choose the HD audio track, even in 2.0. Many of the BRDs I have (mostly opera and ballet, I must admit) have a lossless 2channel PCM track. OTOH, one can convert the 5.1 HD tracks to 2.0/2.1 if necessary.

The big point here is that all this should be entirely digital using HDMI output from the player to the processor, regardless of the number of channels.

That said, if you want the best from traditional 2 channel analog sources, like phono, an analog preamp with an HT passthrough would be a good adjunct to any MCH processor.