The HFB cards are not well labelled inside Volumio; the single crystal Digis (the Digi+ and Digi+ TRansformer) both work with the Volumio HiFiBerry Digi, the twin crystal Digi+ Pro has the correct label. You need the HiFiBerry Digi setting.
The Digi cards feed both optical and RCA outputs at the same time AFAIK, and the fact that you’re getting some output suggests that the RPi side of things is working correctly (probably). If the optical shows a red light when powered up and Volumio running out to it, then the Digi+ is working and selected. Which version of Volumio are you running?
The other thing is that the AVR340 is a receiver that can decode certain things but the limits shown in the manual appear to be 96KHz at the top end. P.39 of the manual says:
When a PCM signal is detected, the Lower Display
Line will briefly show a message with the letters
PCM, in addition to a readout of the sampling frequency
of the digital signal. In most cases, this will be
44.1kHz or 48kHz, though in the case of
specially mastered, high-resolution audio discs you will
see a 96kHz indication.
I’d suggest that your receiver is the weak link in the chain…