Thanks Davide.
I think I have found the relevant information now, that answers my questions: Direct DSD support on Volumio up to DSD512 - Volumio
I am getting not quite the same result as you, Davide
DSD is being resampled to PCM 352.8 for the HiFiBerry DAC+ (no Pro). This is good, as integer resampling to a multiple of 44.1 will use less CPU power on a weak device like a RP than non-integer to a multiple of 48
I tried to read out the samplerates the HiFiBerry DAC+ supports natively using
cat /proc/asound/card2/stream0
Command as used in the thread below. But could not get any information. Does this command only work for USB devices?