Here is a very detailed comparison between the AD2000 and AD2000x (original language is in French):
https://translate.google.com.sg/translate?sl=auto&tl=en&js=y&prev=_t&hl=en&ie=UTF-8&u=http%3A%2F%2Ftellementnomade.org%2Fforum%2Fviewtopic.php%3Ft%3D4550&edit-text=&act=url
As you can see, technically the AD2000x might be slightly better, but the AD2000 sound more euphoric. In short, the AD2000X pays more about technical/analytic aspect while the AD2000 is clearly more oriented for musical enjoyment.