$20 worth of an Amazon DVI to HDMI adaptor + a 1 foot HDMI cable makes for a clean and simple connection for the 150b to an external DAC over I2S. Best to keep I2S cables as short as possible.
For connecting the older Gustards like the R26 to a RS150b you could get a standard DVI monitor cable, cut it down to 1ft and terminate it with that HDMI breakout connector like @StandardModel shows, but swap pin 7 and 9 to get the channels inverted. Should be good to go.