I own the 700, 900, 1k, and decided not to get the 2k after hearing it a few times.
The AD series is great for its soundstage and the feeling of openness.
The 900 can be considered a direct upgrade to the 700, it does everything the 700 does but better.
Starting from the 1k, significant amounts of (lower-mid) bass are added.
The 700/900/1000 can be considered bright depending on what you are used to, but the 2k is a very warm headphone.
To me, the 1k is probably the most balanced and the one to get if it is going to be your only headphone.
In fact, I would consider the 1k and 2k to be in a separate series to the 700/900.
They are not really direct upgrades all the way up the series.
If you are after M50 bass impact, you are looking at the wrong stuff.
The bass bloat and lack of control on the M50 is replicated in the 2k (but to a lesser extent than the M50), so in terms of similarity I would probably consider the 2K to be the most similar. But the M50 leans towards a darker signature where as the 2k is more warm (i.e. the bass bloat is concentrated on a slightly higher frequency on the 2k).
It would appear to me that maybe you would be more satisfied with something like the fostex X00 rather than these....