Pros: Bass boost is pretty well implemented, powerful and good drive capabilities, nice warm sound
Cons: still won't drive everything, E12 regular edition has audible hiss which is fixed in the DIY version
So a couple of friends loaned me the E12 a month or two back and I have been listening them on various rigs and familiarising with them over a while, I'm not that experienced in the portable realm but will do my best to give you a clear picture of the sound(hopefully).
With the design, I find the switches mostly a pain in the arse to use because they are recessed when I use the amp with different headphones, like my hard to drive AKG 240DFs and say the more sensitive IM-70 from Audio Technica. This problem is solved in the E12DIY which sports a gain flick toggle switch at the front panel at the price of giving up all the other features of the regular E12. I really like the knobs on the amps as they still do no feel very loose after the two months I have them and they are also less prone to accidental adjustments due to their design.
Without ado, I felt the Mont Blanc was pretty win(and maybe their only win product in my book) so far, having owned the E11, its a huge step up in sound and direction of Fiio. The soundstage is pretty large compared to the Project Iona I had at the point of review. Comparing the Project Iona, O2 and E12 side by side, the E12 comes of as the warmest sounding of the bunch(though they all measure flat) and also the most technically competent of the bunch. Bass wise, the E12 tops out have a more defined bass body than the O2 which sounded somewhat mushy in comparison. As it has a good amount of power, it swings between volume levels without feeling compressed and strained compared to the Objective 2 which was flat(like dynamically flat) in comparison into harder loads(it still holds true even for easier loads though). The one quibble I have is the lack of synergy/drive with a picky headphone like the HD800, which is probably due to the choice of opamp(OPA1611 iirc) used. It sounded harsh and dry into the HD800 whereas my receiver though noisier, drives the HD800 effortlessly(I noticed the same in other big amps of course) so don't be too tempted to use this into replacing a proper desktop amp. Into the AKG240DF though, I'm simply happy with the combination.
As for the switches, I found the gain to provide a good balance between the headphones(and the feel in-ears) I use. The bass switch is pretty good, boosting the low end of some headphones in a good way without obscuring the mids. For me, the crossfeed though a boon for some was frankly just junk for me, I think I would stick to software or higher end implementations of crossfeed. For the purist though, these added features to an extend kinda harm the sound and is the cause of the hiss in the E12, how I know? Thats because I have the E12DIY which removes all these features and changes the gain switch to a little toggle switch, I love it. For the purist, get your hands on the E12DIY especially if you have sensitive IEMs
Right now, I'm using the E12s as one of my reference amps given my heavily budgeted situation and will update impressions on fitting in some of the better (newer) opamps into the E12DIY and see how it goes. Other then that for the regular headphone users(portable headphones or studio cans) I heartily recommend the E12 and for those more discerning ones with sensitive cans and IEMs, I recommend the E12DIY even if you don't "DIY".
Associated Equipment: Creative SB1240, Stoner Acoustics UD110MK2, Ipod4g w Accudio, HD800 Rabid Dawg, AKG240DF, Audio Technica IM-70