I've always wondered how close they were. A number of M2 owners on AVS claim the M2s are better. I think I recall some "not in the same league" comments even, but I don't believe any have done a proper blinded comparison. I'm not saying the M2s aren't better sounding (even at a volume the 7 series are comfortable playing at), they may well be, but if so what is it in the spins that would indicate that might be the case?
Larger speakers control directivity lower (unless it's a some sophisticated design like ME geithain or d&d)
Also they have a single driver playing 800-20000 without crossover.
705 and 708 sound different even at low level (not on all material, or at least it's not that obvious), and M2 is even more different from 708 than 705
The Dutch and Dutch 8C has two 8-inch subwoofers powered by a 500W amplifier, I think you're the one underestimating the D&D.
But it has a single 8 inch woofer to play from 100 to 1250 hz. M2 needs to play 30-800, but it can be offset to subwoofers if needed. dd 8c also has all electronics and drivers in a non-vented enclosure (less vented at least), I wonder how hot it gets inside if played loud for extended periods of time. Drivers are very ineffective thing, they mostly produce heat really.
I also believe that some studios have quite different SPL requirements as they need to listen to uncompressed material occasionally. People in studios seem to have no problem klipping neumann kh 310 for example.
There's a reason there are speakers like genelec 1236a in production (1 inch compression driver, 2x 5 inch midrange domes, 2x 18 inch woofers).