As audio geeks, we spend a lot of time A/B comparing pieces of gear to choose which one is best for a given task. The key to making quality comparisons is to match the volume levels as exactly as possible. If the volume level isn’t matched, our ears tend to perceive the louder item as being “better” — even if the difference in level is very slight.
For a quick check, using your ears to balance levels may be okay. But for a true apples-to-apples comparison, it’s better to use either an SPL meter or the VU meters on a mixer or some other piece of gear to ensure the levels are accurately matched. The problem is, when setting levels by ear, your perception can be affected by the tone of the signal; if, for example, one is brighter or more/less compressed than the other, if may seem louder, when in fact it isn’t.