How do the phase vs frequency plots of the speakers compare?
To really figure this out measure phase differential or transfer function between 2 speakers over the same frequency range.
Make sure the mic doesnt move and the mono speaker is in the same position for each test.
Axiom has this info Im sure.
A speakers relative phase to a boundary vs frequency or another speaker can throw its perceived location. This is used routinely in mixes to create spacial effects between channels. Doing this intentionally with placement or messing with phase (distance) processing can also create this effect.
