This is actually harder to do correctly than just using the volume controls on DAC and preamp and judging the SPL subjectively.
There was quite a lot of research I believe in the 1970s, which established that very small volume differences misled listeners into always preferring the louder one even though all other factors were identical. These differences in volume could be a small as a fraction of a decibel ( e.g. 0.25dB which is stated by professional recording engineers as the lower limit of a detectable level change by them when comparing mixes. I suspect that average listeners would not necessarily be directly aware of such a small change even though it may register subconsciously).
Hence you should find when reading articles comparing , say, amplifiers in reputable magazines that the testers state that they have equalised the volumes to within 0.xdB.
To do this an acceptable music listening level is chosen on one component. Then , without changing the level, a test tone is substituted for the music. This is necessary as the next stage involves measurements. The voltage produced at the output terminals is measured and, using the same tone, the volume control of the second component in the comparison is adjusted until the same voltage ( i.e. +/- 0.xdB) is measured at its output. The volume control position is then marked on the component ( masking tape is handy for this). Of course if wished this could be repeated at various SPLs to give a range of comparable settings.
Without using such a regime I would be hesitant of judging that a 6V voltage output is necessarily better than a 2V one when producing the same SPL through the system as I doubt that I would be able to set a comparison within 0.25dB or better reliably by ear.