Most likely due to the fact the lower notes overshadow the higher frequencies, making it harder to hear the mid - upper bass range, especially if the room is sucking out some midbass etc. all the while emphasizing a lower range.
Jay hit the nail on the head for why I’ve preferred my M80s alone for music just look at the before curve:
http://www.axiomaudio.com/boards/ubbthreads.php?ubb=showflat&Number=307389&fpart=1Even though the curve for the M80s is going to be no better, w/o the big low bass hump overpowering everything else the M80 come out sounding “tighter” and “clearer.” It’s also why I’ve often run my M80s using “double bass” or “LFE + Mains” to get some smoothing from having multiple bass sources even though they are not optimally placed for that purpose.
