I have the mm-1's and they are magnificent. If you have never heard them, do yourself a favor and go to your local apple store and have a listen, they usually have them. You won't need another dac with them, they sound great as is. They are better than the A2 and the XS 2.1 (never heard the XS Book or the airmotiv 4 though). They just have a perfect sound to me, amazing bass, great vocals and silky smooth treble. They also have a sweet spot (sitting in front of your pc), sit there and you will have a smile on your face.
I also have the HE400 headphones that I have to EQ to hell to make them sound nearly as good as the mm-1's. The bass on the HE400's is kickass though. The mm-1's also have a pretty good headphone amp in them too.
They do have a hissing problem but only if you connect the analog-in and the USB at the same time and play at high volume, with just either one connected there is no hiss. I have tried it on 3 pairs and all hissed so it seems to be a design issue. The hiss is mainly audible when no music is playing but is very obvious when sitting in front of your pc. If you have to use the analog-in and the USB, you will have to pull one of the cables out all the time. Luckily I just use the USB. I don't think B&W will ever fix it because they won't even admit there is a fault. Still easily worth every dollar though, even with this problem. Great sounding speakers!