Can you help me select a pre/pro?

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.