Sorry guys, didn't mean to leaving it hanging but it was only a short listening so it might not be accurate. From what I remember when switching to X5 is mostly the highs.
The highs on X5 definitely stands out more than AK120 II. It's not as smooth and controlled as AK 120 II but more lively, airy and a little jumpy to my ear. In fact I felt the overall sound of AK120 II is more controlled and seemed more balanced nothing stands out.
AK120 II sound resembles a clean tube sound which are smooth, moderate to fast attack and with good impact on low end, sweet mid range and of course warmer with deeper sound stage. Also there's a bit more bass body to it.
While X5 has more kinda bold, a bit more forward presentation and less controlled dynamic and happy trebles. It's the sound that has a "free flowing" dynamic thus it's more lively and exciting not as smooth as AK120 II.
I remembered listening to MJ - Thriller and Billy Jean on AK120 II and it sounded really good..more clean analog feel. The bass line and drums on the song Thriller can and tend to sound mechanical and anemic with "cheaper" source but AK120 II does this perfectly to my ear. Bass sounds full and well controlled and the drums is more real. However, I didn't have a chance to listen to AK100 II nor the 240 II.
When I finally got home, I paired X5 with RSA Predator and put my LCD 2.2 on and ...boom there's the sound of the new AK120 II or at least close to it.