You can’t manipulate audio levels without touching “quality”. The bit pattern will remain untouched, but you lower the amplitude.
The question is, are you even able to detect it, as the change is that minor (a few dB’s).
To test the audio impact of volume changes:
put the software volume control on.
lower the audio level with 60dB
compensate by cranking the amplifier up
Now do the same, but lower the out by 5db. If you’re not biased you won’t now the difference.
Benmuziek, it appears that ‘Replay Gain’ is not directly supported (no UI control) in Volumio … that is a fact, and complaining about it doesn’t alter it. You were given a link earlier, and an explanation of how you can alter mpd.conf (mpd.conf.tmpl for persistance) from the command line to enable mpd to follow ‘replay gain’ tags.