I can't completely answer your question, but I do have a tid bit of info on the subject. The DR stands for David Reich, who was one of, or the designer of the preamp. I believe that after he left the company, they dropped the DR from the model name. I had a DR 4 and Reich's signature was printed on the circuit board. Incidentally, I tried out a DR 6 and actually preferred the DR 4 in my system. The 4 sold new for $1600. The phono section, however in the 6 was a bit superior.
