I am still listening. I have run the 400i for close to the recommended time . 150 hours will be finished tonight. I have listened for maybe 25 hours. Next week I have some time off of work, so I will try to spend some time actually comparing them to some of my other phones.
A few things to mention until then.
With some quick casual listening, I prefer the HE-400i to any of the HD580 (modded), Hd600, or HD650. If they stay here I can see them being my go to phones for general listening.
I find them to have a very pleasant sound. One that makes me want to come back for more. The mids for vocals is simply amazing compared to what I have been listening to. I have listened to a bunch of female vocals, (Alison Kraus, Celine Dion, Natalie Cole, Lady Antebellum) that always were problems with the HE 400. They sound (400i) so much better on this material to my ears!
They do not have the "air" that the 400s do, but this is a matter of balance I think. The top end sounds much more real to me. Symbols, bells, and chimes give a much more convincing presentation than the 400.
Bass... I am not sure why they would be considered bass shy. I have listened to a bunch of organ, upright bass and synth with good bass. When the bass is in the recording, the 400i plays it well. Again, I feel like the big difference in the perceived bass between the 400 and 400i is balance. I am not a bass head, but my setup with speakers will do 115 db at 15 hz. We are talking 4 15" subs and 6 10" in folded horns bass tuned to the room. I do like bass, but do not want or need it to be intrusive.I have done all I can to make the system sound real. I ... and it is personal taste, find the bass on the 400i more pleasing than that of the 400.
Soundstage. I have yet to find any phone that has much in the way of a large and realistic soundstage. I find differences generally pretty small and difficult to detect. I can say that while listening today there were times when using the 400i that I actually turned my head to see where a played sound was coming from. This when I was really not concentrating on the music. This impressed me. I attribute it more to the naturalness of the sound then some large soundstage though.
I will try to write up something more on direct comparisons with the Senns (etc.)and analytical listening next week.
BTW, I was listening to some Kansas, Emerson Lake and Palmer, and Alan Parsons Project earlier, portable, Fuze to C5D... I liked what I was hearing. I think they do a credible job with all of the classical rock stuff that I have thrown at them. Pink Floyd, Carlos Santana (Blues for Salvador is very nice), Grateful Dead ...etc.