Hi Sloped,
The short answer is that deep bass starts becoming directional above 80 Hz, so you don't want to feed bass frequencies above 80 Hz to a subwoofer or your ears will "locate" the bass at the subwoofer. The M3's output at 80 Hz remains strong and is similar to its upper bass and midrange output.
With most compact 2-way speakers like the M3, the default crossover setting of 80 Hz is almost always the best choice.
If your subwoofer is at the front of the room near the M3s, it wouldn't matter much if you used a higher crossover frequency. And to kcarlile, yes, the hump in the M3's lower bass is still there--it was apparent in the last set of double-blind tests I did when the M3s were compared to a competitor's pair of floorstanding (!) speakers that had virtually no bass output.
Regards,
