I just got my hifiberry MiniAmp and installed it on a rpi3b which previously had a hifiberry dac+ connected to it. I changed a line to dtoverlay=hifiberry-dac and the miniAmp worked like a charm.
The only problem now is that I have no option to change the volume. With the dac+ I could change it with "amixer sset Digital 76%" but that doesn't work anymore. I have seen a couple posts about creating a software mixer but that also does not seem to work for me.