I am not sure you have that quite right. The external PSU powers the master clock analogue and analogue stages and converter circuitry. The clock oscillator is coupled into the digital circuitry through the iCoupler, but crucially is powered within the analogue circuitry demarcation. The critical digital signal chain including through the SHARC and BlackFin DSP processors are powered by the Naim DAC PSU when the analogue element is powered by the XPS2.
This diagram from Naim illustrates it either side of the analogue/digital dotted line.
