Thanks Mojo!
I did indeed attempt a near-field listening test by covering the top then bottom tweeters individually while listening to the other. The difference wasn't slight with the lower tweeter seemingly producing generally higher-frequency sounds.
The proximity to the uppermost midrange speaker may indeed color my perception enough that it's impossible to make this sort of comparison. When I have a bit more time tomorrow I'll try it again with a few more sources and for both the left and right speakers. I checked only the left tonight because I thought I heard just a bit of harshness in it that wasn't present in the right though that might well have been the source rather than the speaker. I suppose if I should encounter it again the most reasonable thing to do would be to swap the upper and lower tweeters to determine whether the difference I believe I'm hearing is position or speaker dependent.
FWIW I love the M80s. Astonishing clarity to my ears but then again I wouldn't know an audiophile speaker if it bit me on the ass.
