I purchased an RS6 to try the R-2R DAC and was very happy with the music that it produced. I decided to try an RS8 to see if double the R-2R components would be twice as good. I can say there is a difference between the Darwin I and Darwin II DACS, but not twice as good (typical diminishing returns). For those that want to travel with their DAP the RS6 is the way to go, it is much lighter and more portable. If you must have the best of the best, the RS8 is your DAP, it is a great sounding TOTL DAP and makes you want to keep listening to the music. However, it weights a ton, with all the extra components and the huge battery. Both units have the magic to make the music enticing, but I think the RS6 is the better value for me. Which ever way you choose to go you are a winner.
The RS8 is heavy, so not super portable. All the extra components compared to the RS6 does make a sonic difference, but you need to break in the unit for at least 50 to 100 hours to get the best performance. The balanced Po output has plenty of power to drive my Hifiman HE-1000SE. As a source connected to my Woo Audio WA6SE tube headphone amplifier, the combination is pure magic. And by the way, the RS6 is a great portable version of the R-2R DAC DAP.