Hi, I have no experience with the M1200's but I did have the M700's. I used these to drive my 4ohm Kef Reference 5's. Although these have a reasonably high sensitivity I do play them quite loud and the M700's coped admirably with plenty more to give. I was so impressed with them (and PS Audio) that I wanted to upgrade, the 1200's would of been a natural progression but I jumped straight to the BHK 300'S - no regrets there. I suspect that the 1200's will have oodles of power for your D3's. My advice is don't get too hung up on specs, they can sway you away from making good decisions.
cheers, Mark