First whomever said R2R DACs are the flavor of the month is completely wrong. We build both types of DACs Sorekris R2R modelue and Sabre 32 Bit. THey are differnt sounding but the R2R are not soft sounding in comparison. They are definately more musical and less forward sounding.
Part of the problem like most is that you are swapping out DACs they are similiar in design. To design a componnet that either is tube versus non-tube is the first question. The most important design feature is the power supply. We build DACs that have 30lb power supplys for our tube designs. That is where you get the main differnce in sound. That is where you begin to get reference sound from.
Stop looking at one feature of any component and begin learning what makes them sound different. How do you get a beautiful soudning mid-range, how do you get layering, who do you get a big soundstage, dimension, tone, bass definition,etc.
Happy Listening.