I had the chance to hear and compare Eric Clapton's "Change The World" using two setups:
1) iPad headphone out -> ATH-M50
2) iPad camera kit -> Fiio E7 -> ATH-M50
On several attempts the difference was very hard to find, but I think I managed to feel some of them (I could be wrong!)
1) The M50 already does a good job on its own, and the sound is very clear with or without the E7 - I can hear all instruments. At first it sounded like a perceived difference in EQ setting, but I think I need to articulate it better. With E7, the mids (ie. piano and guitars in the background) appear generally weaker (but not significantly), but in sections with strong mids the E7 improves the 'attack' and this might be one reason why the vocals sounded better here. As I increase the volume on E7, the amp nicely restores the weak mids (I would say up to 80-90% becomes comparable to mids using headphone out). Sometimes this characteristic may be good for songs with very forceful mids.
2) Hi-hats and cymbals are more controlled with E7, making it easier to listen to the song at louder volumes. I think it can still do better in this department, I needed more presence here.
3) Bass is a little more confident on the E7. For bassheads, I think the Lv 1 bass boost setting is going to be a delight. Lv 2 and Lv 3 are too much for my taste.
4) E7 provides better instrument separation (The effect is more apparent in Gwen Stefani's "Hella Good"). From time to time, some instruments will try to 'sing' and attract your attention. I may be exaggerating, but I guess this is an area where the better amps might outperform E7, and this is also a clear area where E7 outperforms E5.
5) As others would say the soundstage is a little wider (but I tend to think this is due to the better instrument separation).
Overall, the difference isn't something a casual listener might appreciate, but the slight (not tiny!) improvement for my M50 is a very welcome one. I agree with a lot of people who posted here that it's definitely a steal (that is, when compared to competing amps which are generally expensive). In most (if not all) cases, I prefer music on my M50's with the E7.
Anyone with experience on E7+E9 paired with M50? I wonder if there's still gonna be improvements or if it is just louder (?)