I wanted to hack something around this, since my amp, speakers, speaker placement, room and listening position are constant. But I have no idea how to actually go about this.

I can fetch the pre-amp level, but the actual sound level will be dependent on the source's actual volume which isn't constant (see: loudness wars). I could react according to a "measured" level, but how should I deal with a quiet portion (think classical music)? The closest I came was to use replaygain, but then that won't work with spotify...

I wonder if anybody came up with a solution to this.