coax vs rca for older cd's

Your terminology is somewhat confusing. Coax meaning digital PCM? RCA interconnects meaning analog? If that is the case, I would guess that you probably would do better with a digital connection, because it seems DAC technology has made some leaps and bounds in the past few years. Frankly, you might want to connect it both ways. Switch back and forward and see which type of connection you find more musical.

If your pioneer elite is older then using the digital, coax, output would basically turn it into a transport and by doing this use the dacs of the b&k. As it is a new ref. 50 I would most certainly do this. IMO.