Here is a review of the JBL's, complete with a FR curve and judging by the work the reviewer had to do to get a neutral sound, I would take the M80s. That FR curve and the review stating the JBL's sounded bright, leads me to believe the sound is bright because of the dip in the midrange from 200hz up to about 4khz. I would guess this dip is due to the one extra tweeter (of different composition?), compared to a single mid driver, but having never heard the JBL's this is all speculation on my part.
All I know is the M80s are a spectacular sound for the price.
