These are very different sounding preamps. The Ref 2 mk2 was the first Ref preamp to use the 6h30 tube and is more neutral sounding. The Ref 1 uses 6922 tubes and has more of a classic tube sounding but is still not tubey. Preamps have come a long way in the last 20 years or so, far more than amps. I would go with the Ref 2 mk2 if it were me. But I really like the more detailed and neutral sound of the 6h30.
I have heard the Ref 5se with .8 Pass amps and it was a nice match. Pass makes some excellent equipment.