You are supposed to use a sound meter at your listening position, use dB(C)weighted Leq (a time integration of the sound rms loudness performed by the measuring instrument) then play your reference audio material to adjust the amplifier or DAC output gain until you read 77 dB(C) of sound pressure level, both speakers driven
But you could actually set the loudness level at another point here 60....90 dB, as long as you consider to further compare the audio to that relative reference
That say, any tracks showing around 0dB slow rms when referred to the same K initial metering should sound nearly equal in loudness
Using dB(C)weighting makes sure to properly assess the bass content of the audio, but take care about the acoustic coupling of both your listening room and speakers, that could induce a subjective but significant difference with K metering looking similar ed:
Hope it helps :-)