Likewise, I cannot completely answer your question, but can offer some insight. I also purchased a Classe DR-4 and had it upgraded to the newer Model Four. Thus, the Model 6 is newer. Sonically, the DR-4 was warmer and smoother. The Four was airier, had a slightly larger soundstage and I guess could be called more "neutral", whatever that means. It was not as warm sounding as the DR-4, but was more revealing. These are not big differences but they are noticeable.
