In case helpful, I edited /flash/config.txt and added the line dtoverlay=hifiberry-amp
For good measure, I also added dtoverlay=hifiberry-dac and dtoverlay=hifiberry-amp4
The Hifiberry-dac file did appear in System » Audio, but selecting it does not appear to produce audio output, although the advice from Hifiberry is that Hifiberry DAC drivers usually work for their Amp.
Only use the first line, and answer my question. Background: RPi5 needs more wattage, compared to RPi4.