I love my AD900, but I recently got MS2i and I have been using them at work for the past few weeks. After owning the AD900 for a few months I really wanted something that I could rock out to, and the MS2i sure do deliver. AD900 sound really nice and polite after hearing MS2i all day at work
I haven't put much time on the AD900 lately (haven't had much listening time at home recently), but I think that the Alessandro do complement them well. I like the AD900 for a lot of music, particularly classical, daft punk, radiohead, and more. They are good too for rock and metal in that they have good detail and an even sound overall. But to really rock out and toe-tap to some DT, Tool or Opeth, it's MS2i all the way
Keep in mind that Grado/Alessandro are pretty detailed and crisp and some people don't like the sound (or just can't handle it
), but I find them just fine. In terms of bass, I'd say they're probably similar (but haven't compared side by side) to the AD900; the bass is all there, well refined and punchy (especially when amped) but isn't accentuated over the other frequencies.
The MS2is have replaced my PortaPros for listening at work, which are also a fun can, but I found the bass too much to really hear enough detail and this decreased their rockability for me. Still great for something like Justice.
Just my experiences, good luck in your search.