After going through 3 pre-pros, I've ended up with the D2, and I love it. It is one of the few components that I have no desire whatsoever to even consider changing.
The automatic room correction is amazing. This is where it charts each speaker's freq response, and automatically corrects it for the room. This is a "cant live without" feature in a pre-pro as far as I'm concerned.
I can't comment on the Ref70, as I've never used one.