Always fun and interesting doing speaker comparisons, but be sure and get the loudness levels exactly equal when you switch from one pair to another. The bias towards picking the ever-so-slightly louder speaker is incredibly persuasive and ever so subtle. I have to constantly check levels with the ol' Radio Shack SPL meter when I do comparisons.
And try and place the speaker pairs in exactly the same location. If you are switching with a comparator, sometimes just the slight shift in soundstage height will bias you to prefer one speaker over another.
A really ruthless test of spectral (tonal) balance is to line up a single sample of each speaker and do instant comparisons in mono with one speaker of each pair. You can zero in on tonal differences very quickly without the flattering and in some ways masking effect of stereo.
Axiom Resident Expert