The RS-2 seems disporportianately not as good as the RS-1 from what I'm told, but I don't actually have much experience with those headphones. If you can come by a used one, you may consider a Grado HP-2 since it has a flat response and very fast driver that has been called "Stax-like" by many. Still, I hate to recommend a headphone no longer in production.
It seems that the Audio Technica ATH-A100Ti is one worth considering, according to what Dusty and Hirsch have to say about it. If it carries over any of the good qualities of the expensive W2002 without the weird coloration, it could be a good option--but hardly anyone beisdes them have heard it yet.
Most people here will tell you that other than the Grado HP line, the Etymotics are the closest thing to Stax due to their neutrality and flat frequency response. These are in-the-ear type headphones and may not be what you're looking for but you should consider them if you really want Stax-like accuracy. The real advantage to them is that they're closed design and block 25db of outside noise so they're perfect for very noisy environments.
Undoubtedly if I don't say this, someone else will--even though they have a slightly recessed upper midrange, many jazz fans and jazz vocal fans love the way Sennheiser HD600s sound. I don't feel they have anywhere near a fast or articulate midrange as the Stax so I don't know if they meet your requirement, but they're such a high end standard around here that I'd at least try to audition them. If you end up buying a pair, order the Cardas HD600 upgrade cable for them--it makes more of a difference than you'd suspect. If possible to audition, also try the Beyerdynamic DT931 and see what you think of that one but it also won't be as fast or accurate as you may be accustomed to.