I emailed AT directly asking about the M50 in relation to the rest of their line. Here's the reply I got. It sounds promising.
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Hi,
The only immediate differences I see between the M-30`s and the
M50`s is a slightly larger drive (45mm in the M50 versus 40mm in the
M30),a wider frequency response (15-28,000Hz vs 20-20,000Hz) and a lower
impedance (38 ohms vs 65 ohms). I have put in a request for more details
regarding the drivers and a request for a frequency response graph. I
can tell you I have heard the M50`s and they sound better to me than any
of our headphones, regardless of price. I also like the M30`s but have
not listened to them for several months and I did not have the
opportunity to compare them side by side. The 50`s have this huge,
aggressive sound with the best bass I have ever heard through a
headphone. I will provide you more information when it becomes
available.
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It's pretty slick for a studio monitor, which are usually ugly.