JBen on the Planar Asylum has measured the MMG's both ways and from the back the frequency response is tipped up between 10 and 15 kHz:
http://db.audioasylum.com/cgi/m.mpl?forum=mug&n=144728&highlight=MylarFront&r=
I'm guessing that's what you're hearing.
http://db.audioasylum.com/cgi/m.mpl?forum=mug&n=144728&highlight=MylarFront&r=
I'm guessing that's what you're hearing.