Volume can be deceiving
When audio gain is boosted it can be perceived as sounding good or better. However this is not always the case. For example when EQing a final mix at the mastering stage we are cutting and boosting the gain of various frequencies. If we boost 8 kHz with a high shelf thus boosting everything above 8 kHz by 3db it may sound good even pleasing. However we must remember that boosting on a final mix-down is like boosting that frequency on all the tracks that made up the final mix. It sounds pleasing because of the volume going up but we may have done more harm than good. When things are louder it perks our auditory sensors giving us the illusion of it always sounding good. Therefore we must do a/b comparisons at the same gain. Meaning the processed mix must be that same volume level as the unprocessed mix in order to make a fare comparison. So for example if the processed mix has an increase in gain due to the EQ boost that was made, the output of that signal must be reduced to match the unprocessed signal, then and only then can you make a fare comparison between the unprocessed and processed mixes. Gain matching is critical when making any adjustments whether it's EQ or compression during the mix stage or mastering stage.