I've been sifting through some of Gordon's posts (he is the iqaudio designer) over at www.volumio.org .. maybe the below quote is an additional clue .. in any event, this board is getting a lot of praise ref. SQ

The IQaudio Pi-DAC+ and older Pi-DAC use the PCM5122 dac which has built in hw volume control.
usage of this feature requires i2c commands to be sent to the dac to adjust the volume.
This can be done using direct i2c commands or via ALSA function calls. Also code for the IQaudio DAC for Rotary encoder is available on www.github.com/iqaudio/tools

Very best regards,
Gordon