First off we have not seen any test DATA on the M2, second there is something going on with that predisposed bump in the low end with the LMII/LMIII and it's definitely not flat, and third I would want to see some more testing done by other folks before I believed any claims.
Bass Gear Magazine performed their own tests, as far as I can tell, and the curves looked just about the same as the data from Markbass. What is this low end bump you speak of? The plots look entirely flat inside the audio spectrum.
Also there is more than one way to look at the signal and proclaim it is flat. YMMV
Please enlighten me. If you make the assumption that your amplifier is more or less linear, the amplitude vs frequency plot pretty much tells you what you need to know.