To achieve +6 db you need to quadruple the power.
705 = 101dB @ 1m
708 = 108dB @ 1m
But both powered versions of these two have the same wattage.
Considering these are intended for production work, it would seem the primary difference in design is to allow for near field or far field. For HT, the 708 should be able to compete with most floor standing speakers, which certainly increases their value in that they can be wall mounted, plus each speaker can be individually calibrated with the built in electronics.
I've heard the M2 and honestly I hate the 7 series. Not because they are lousy performers, quite the contrary, it's that they encourage me to abandon DIY speaker building.
Assembling an HT with three 708's up front and the passive 705's for surrounds is actually less expensive than my 3 and 4 way DIY designs - have only built a 3 way center so far.