HDCD encodes the equivalent of 20 bits worth of data in a 16-bit digital audio signal by using custom dithering, audio filters, and some reversible amplitude and gain encoding (cit. en.wikipedia.org/wiki/High_Defi … le_Digital)
The problem is that when ripping HDCDs the PCM samples of 16 bit are stored in the file as they are, undecoded. Most DACs do not recognise the special HDCD encoding, thus not giving the user the extra dynamic promised by the HDCD format. Even worse the peaks are clipped and the sound sligtly distorted (see audiomisc.co.uk/HFN/HDCD/Enigma.html)
A nice feature for Volumio would be to recognize such format and perform the HDCD decoding on the fly, thus sending the “uncompressed” PCM stream 20/44.1 to the DAC.