There is more air inside the M3

I would expect the M3 to weigh more, although...

On the Canadian site the weights are different; I don't know which numbers are correct :

http://www.axiomaudio.com/en-ca/bookshelvespecs.html

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8