To avoid confusion, you are talking about this Douk Audio?
Douk Audio U2 PRO XMOS XU208 USB to COAX/OPT/HDMI Converter Digital In– doukaudio
That is not a DAC is is a USB to COAX/Optical/HDMI converter. It is a digital to digital converter and a DAC is a digital to analog converter. Like others in this thread already said, you should buy a DAC. Correct me if I have pointed to a wrong link and you have another device.
This is probably because the TOSLINK input of this receiver only support up to 96kHz PCM…
The company does not publish the supported sample-rates of the TOSLINK input and the USB 2.0 input on the front of the device…And it does not support DSD…
Your best sound will come from an external Digital to Analog Converter (DAC) connect via the AUX input of the receiver using good analog interconnects, as @bitracer and @OffRode have already pointed-out…
What you have is a Digital to Digital Converter (DDC) if you had another DAC or digital device that had inputs that supported the output of the Douk DDC, things would be different…