Keep in mind that I only briefly listened to the UE4000's at the Apple Store on their own iPod Touch's, so my impressions may be "iffy". I mostly listened to Linkin Park and Taylor Swift songs to get a good feel for bass punch (kick drums), midrange (guitars and vocals), and highs (cymbals).
Compared to the SRH940, the UE4000 (and I suppose the passive mode 6000 and 9000) definitely has much better bass punch and extension. The large bass presence kind of bled into the midrange a bit, but not too much. This may factor in to the very warm sound signature it has compared to the analytical SRH940. The mids were nice and smooth overall and a bit forward, more so than the M-80 based on memory. The highs seemed rolled-off to me since cymbals didn't have that "ring" after the initial hit. It definitely makes the treble less harsh compared to the SRH940. In terms of overall resolution and detail, there's no comparison; the SRH940 destroys the UE4000. I couldn't really compare the bass texture since the Apple Store was busy with people/employees running all over the place (huge line out the door for the iPhone 5), but the SRH940 definitely has better detail retrieval for the mids and highs. In terms of portable musicality though, I actually liked the UE4000 because of its warm sound. Without using an EQ, the SRH940 sounds very harsh while riding on a bus.
Overall, the UE4000's were very impressive from my initial listen, and YES I definitely like them more than the Amperior because of the non-sibilant highs. For detail retrieval, I think the Amperior does a better job, but the UE4000 was still pretty good from the tracks I sampled.
I'm not sure I would use the UE4000's on-the-go though, since I do like to hear details in the highs but I would say it's a very very good headphone overall.
Holy moly.......the UE4000 is only $100 USD! (Apple Stores don't have headphone product info at their headphone stands.....)
I might just pick one up. For $100 I think the UE4000 is an absolute steal for your money. Definitely leagues better than any celebrity endorsed headphone at the Apple Store, and better than some of the Harman Kardon ones too.
Thanks for the comparison. If you have a chance, please let me know your thoughts on the UE6000 in passive (especially if you have a portable amp) since that should be a noticeable step up sonically and the model I am enjoying right now.