I am using my new Amp+ with a RPi3 and am loving it. Volume control works fine with alsamixer. Normally however I am using shairport-sync, and unfortunately the hardware volume control does not work with it. The reason is that, unlike the DAC+, the Amp+ does not expose a dB volume scale range. The error is:
the hardware mixer specified -- "Master" -- does not have a dB volume scale, so it can't be used.
More info at https://github.com/mikebrady/shairport-sync/issues/232
It would be very helpful to have all the Hifiberry products expose a dB range. The ALSA call is: snd_mixer_selem_get_playback_dB_range.