[Bluemoon]

Like you say, unlikely to be phase, would sound harsh and empty, not dull. What monitors are they, and are they active or passive, if so what amp? Don't forget that when you pan a signal in/out the relative db per channel increases up to +6db* therefore whichever direction you've panned will be driven harder. One reason why kick/bass always centre panned, so transient energy evenly spread and you don't blow your drivers.

*Depending on your mixer/monitor set up.