Your idea of switching speakers is a good one.

One thing you can try is playing a signal that is the same in the left and right channels. Radio inter-station noise is a good one. Measure the voltage on each speaker. If it's the same, your amp is sending out a balanced signal. If the noise isn't focused in the centre, it could be one speaker is slightly more sensitive than the other or it could be your room.

Keep notes. If the same song presents imaging in different ways from one listening session to the next, try to note what may have changed. Could it be volume or temperature?

The Axiom tweeters use ferrofluid for cooling. If your room is cold one day and warm the next, imaging and other sound qualities can be affected. I can tell you when I had v2, I detected sound changes I could not readily explain other than perhaps the temperature of my environment. With v4, I don't hear such variations.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated