Preamp dillema for McCormack Ultra Platinum

Well, there's a Hegel P20 and a Pass X2.5 available here now in your price range that I'd think would both be a nice improvement in refinement over your Ref50 (and at least get rid of the unnecessary ADA processing -- EEK!).  For a little more and moving to tubes there's an Audio Research LS17, or for again a little more there's a VAC Standard LE that could be a really nice sonic match for what you're looking for.  Obviously check that these work with your DNA1 -- I'd look for a bit under 10kOhms output impedance just to be safe.

Just FYI and FWIW, I'm looking at a Sachs SP14 (about $2k stock) or a Backert Rhumba 1.2 (one here now for $2400 I wish I could buy -- dammit).  Looking forward to what a little tube magic can do in front of my SMc amp.  Anyway, hope this helps and best of luck. 

What aspects of sound reproduction are most important to you and what are you looking to improve upon with a new preamp?  Will be easier to make recommendations once your particular tastes and goals are better known.  Also, I seem to recall modified McCormack amps may have a relatively low input impedance, and if so not all tubed preamps will work well with your amp (I could be wrong on this though). 

I had a stock RLD-1 in my system on a demo feeding my DNA 0.5 Rev A, and although good I found it to sound a little veiled.  I think your amp deserves better, and frankly I think a $1500 budget might be a bit low for your amp -- a good preamp is critical to getting the most out of your amp so don't cheap out there.