GC, my first thought is along the same line as one of Casey's: don't try to evaluate relatively subtle sound differences between speakers in the same line by interpreting small variations in frequency graphs. As to 1., sensitivity, this is typically measured by using an average in the mid-range from about 200-2000Hz. The two appear to average about the same and Axiom rates both at 89dB for 1watt/1meter.
On 2., I'd be inclined to think that what you describe wouldn't be enough to make the M50 audibly "brighter".
3. Since the M60 uses the same two woofers in a somewhat larger enclosure than the M50, there's at least the potential to tune the enclosure to a slightly lower frequency and it'd be surprising if this wasn't the design. Axiom does rate the M60 2Hz lower in the specs. Note that the anechoic chamber isn't accurate below about 80Hz(one reason for measuring high in the air on the tower)and so especially in the bottom two octaves, comparisons can't be relied on.
