After reading glowing reviews of both of these in ear monitors I was still unable to decide which one would be right for me so I bought both, first the Shure and a few weeks later the Sennheiser. I already own the JH Audio 16 Pro, Ultimate Ears Triple.fi 10 Pro, HiFi Man HE-500, Sennheiser  HD 650, Shure SRH940 among others. My music files are all in FLAC format and I play them through an Astell & Kern AK120 and also used a Samsung Galaxy S4 for comparison purposes. To boost the signal for some of my headphones I use an ALO Rx Mk2, a Schitt Lyr, and a JDS Labs O2 headphone amplifier.
 
I begin this review with the following disclaimer, since we all have different tastes in music and how we like it to sound, there is no universal "right" way for all music and sound gear to sound. I will try to convey how I like my music to sound and why I preferred one of these fine IEM's over the other though they are both excellent in their own right. I used to work in a recording studio in NY where I was taught that it was our job to capture the sound from the room behind the glass or auditorium as if it was a picture. Get as much detail as you possibly can without altering the overall sound signature while eliminating as many artifacts as possible that could distort or mask some of what we are attempting to preserve. So, I prefer my music to sound as close to "being there" as possible, I want to hear everything that was on the original recording with nothing added and nothing taken away. This of course is a difficult goal to achieve as there are always sonic imperfections inherent in music reproduction so we all make compromises when purchasing gear.
 
The standout area for the IE800 is what I call the "presence" area - that part of the lower treble that makes voices and some instruments sound more (or less) "alive". Being a vented, open-back design, the IE800 have a wider more convincing soundstage than the SE846. They have 2 vents, one is for the sound and the second one is to equalize the air between the eardrum and the outer ear, so the pressure goes away after you insert them. Although they are open backed in-ear headphones, don’t worry as the people around you can’t hear what you hear due to the small size of the vents. Yet, it is their unconventional single dynamic driver design coupled with their vented ceramic non-resonant enclosures that gives them their remarkably spacious soundstage especially for an in-ear model. They are breathtakingly clean with deep, extended, punchy bass response which never impinges on or overrides the rest of the music which to me is amazing. On the high end, the treble sparkles with clarity and incredible detail without a trace of stridency or harshness. That said, the mids, one of the strongest points of this headphone, are excellent  and all the instruments and voices have a good, natural and well textured presentation which surprised me given how deep the bass presentation can be when it is present in the original source material. Up until I listened to the IE 800's, I felt that the HE-500 Planar magnetic headphones from HiFiMan coupled to the Schitt Lyr headphone amp had the widest, most realistic soundstage I had ever heard - not so anymore!
 
Contrasting the IE 800 with the Shure SE846, I found their bass to be somewhat exaggerated especially in the mid-bass region which in my opinion overwhelmed the midrange at times. They reminded me of the Ultrasone PRO 900 S-Logic Surround Sound Professional Headphones  that I used to own but sold because although they had powerful bass, it came at the expense of the upper midrange. The other area where I really found the Shure's to be lacking was in the treble, especially in upper end. Compared to the Sennheiser's they sounded veiled or muffled however, before I actually had the Sennheiser's, I didn't realize how much the Shure were lacking. As I already own and frequently use the JH Audio 16 Pro IEM's, I compared them to the Shure and there too, the Shure came up short. The JH-16 Pro, also a multi-armature IEM blew them away for just a few hundred dollars more however, both have that "congested" IEM sound when compared to an open backed planar headphone like the HE-500. The JH-16 Pro's have a more extended treble with far more detail and transparency and when it comes to the bass, the Shure IEM's seemed to overwhelm the midrange while the JH 16's have deep extended bass that does not mask or in any way obscure any other portion of the music. The only negative to buying custom IEM's is that they are yours forever as they will only fit your ears so you can't ever sell them if you decide you no longer want them - something to consider.
 
What truly amazed me about the Sennheiser IE 800's was the openness, the spaciousness of their soundstage, coupled with the speed and detail they amply provide. They can even compete with the HE-500 amplified by the Lyr with custom pure silver cables and that to me was amazing. They have a low impedance of 16 Ohms  across their frequency range and are very efficient so they can easily be driven by any portable devices and don't require amplification to sound great. What these IEM's coupled to a good digital audio player (DAP) provide is an amazing high end sound system that you can fit in your pocket. Although they work very well with phones and other mobile devices they really shine when coupled with a high quality DAP so if you really want to hear all that the IE 800 have to offer, use a good DAP or DAC and play lossless files otherwise save your money and buy something like the UE Triple.fi 10 Pro for half the price.
 
The comfort of the IE 800 is really good and superior to the Shure. The IE 800s are super small, incredibly lightweight and the least intrusive of any IEM's I have used. I can literally keep them in my ears for hours without any discomfort in fact, I sometimes forget they are in there! Another quality of the IE 800 that I really appreciate is that they sound great even at lower volume levels which for me was a big surprise. Typically, I tend to enjoy listening to music at higher volumes because it just seems to sound better that way. The reason for this is that we perceive different frequencies, especially the deep bass and treble at higher loudness levels then we do midrange frequencies, which was first discovered by Fletcher and Munson in 1933. This is commonly referred to as the Fletcher–Munson equal-loudness contour which means that a listener perceives a constant loudness when presented with pure steady tones at different dB levels across the frequency response. Sennheiser designed the frequency response of these IEM's to compensate for this and therefore, they sound good even at lower volume levels which is a big plus.
 
I really thought of finding some cons to these headphones but other than the price I couldn’t find anything. The non detachable cables are not an issue for me as they are very well made and appear to be quite sturdy. They are more microphonic than the Shure cables however I position the IE 800's with the cord up so I can wrap it over my ears cancelling out their microphonic tendencies. Given the very small size of the earpieces and the fact that they are solid ceramic, it would be nearly impossible to install removable cables without increasing their size and weight as well as altering their sonic character. BTW, I would suggest buying these from an authorized dealer so that your warranty is valid given what they cost.
 
All in all, the Sennheiser IE 800's, connected directly to my AK120 are now my favorite headphone/IEM's, they're smaller, lighter, more comfortable to wear for extended periods of time and if I had listened to them before purchasing the HiFiMan HE-500 and the Schitt Lyr, I would be $1200 richer!

INTRODUCTION:
 
I have owned the AK120 since September and listen to it every day, several hours a day. I use a variety of headphones depending upon where I am. When I'm home, I prefer my HiFiMan HE-500 Orthodynamic cans or my JH Audio 16 Pro and when I'm out I generally use my closed back Shure SRH-940, UE 5 Pro or my custom fit ER4 MicroPro. I have used it with a variety of amplifiers though it really doesn't need external amplification to drive any of the headphones I listed to a reasonable volume level. I was in fact surprised to discover just how good it sounded with the power hungry HE-500's and with all the other headphones, it could drive them way beyond my listening threshold and I like my music loud. The HE-500 being Orthodynamic do benefit from some extra amplification however, they are quite good just with the AK120. When I'm out I leave my amps at home and carry my AK120 on my belt in a small case, the Swiss Gear - Jasper Small Camera Case-Black, is a perfect fit and adds extra protection, available at Best Buy for $4.99 (Model: GA-7876-02F00).

Battery life has been surprisingly good as I generally use it daily for at least 1-2 hours, sometimes more and only need to charge it weekly so I figure I'm getting about 12 hours of listening time and haven't run it down to zero once. The UI is really intuitive, very responsive and easy to use. Even though I have the unit almost fully loaded with FLAC files, about 150GB so far, it still responds with little lag. I have both microSD slots loaded with SanDisk Ultra 64GB micro SDXC cards with the factory exFAT file system intact and the AK120 see's all of my music files and metadata with no problems whatsoever. I loaded my music files onto my cards through Windows 7 by just copying the folders onto the SanDisk cards using a USB3.0 card reader and it worked flawlessly.

Another benefit to the AK120 is the fact that it connects to my computer's USB port (USB2.0 only so far) which allows me to use is as an external DAC to listen to all the music I have stored on my computer and it sounds incredible through my headphones. I also connect it to my home theater system directly from the AK120's headphone jack to an aux input and the sound is again - amazing, better than my DVD player and I no longer have to look through the hundreds of CD's I have to find what I want to listen to. I have to say that I was skeptical before I bought it because it costs so much and I wondered if it would indeed prove to be worth the expense. Given Amazon's great return policy and the fact that there are not many others in it's class to choose from, I decided to try it and am very pleased.

FEATURES:

The Astell&Kern AK120 measures 2.33"W x 3.50"H x 0.57"D and weighs only five ounces. It's main interface is through a sharp and responsive 2.4" QVGA IPS Touchscreen while on it's right side it has a analog volume control wheel and on the left side, three tiny control buttons for back, forward, and play/pause. All the controls function fully even when the screen is off and a brief touch to the power button brings back the display while another brief touch darkens it again. Across its top are the earphone/optical-out port, an optical-in port, both 3.5mm and the power button. On the bottom you will find a micro-USB port and a sliding door which covers the slot for the two microSD cards. It also comes with an well crafted Italian-leather carrying case that has cutouts for all the aforementioned controls except the microSD slot which remains covered. The device can be used at all times while in its protective leather case only requiring removal if you need to change or add an SD card.

The dual-DAC AK120 holds 64GB of audio files via its internal memory and can add 128GB more via two 64GB microSD cards for a total of 192GB (180GB actual). It's equipped with two Wolfson WM8740 DACs each running in mono-mode that handle virtually any  format including DSD via new firmware released in July and has a nifty, ergonomic design that's attractive and simple to use. Best of all, it sounds great with all types of music, its dual DACs providing fabulous resolution, speed, and great sense of space. The warranty is for a year and as the company has a service location right here in the United States, you won't have to ship it back to China for service and if you have any questions, you can call them at (949) 336-4540. BTW, I have no connection to the company and bought my unit from Amazon on August 31, 2013.
 
SOUND:

Now for the all important question, how does it sound? In a word - amazing, it has a wide soundstage, is unfailingly rich and resolving with clean non-sibilant highs, a gorgeous midrange and deep, well controlled Bass (when it's present in the source material). That said, the one thing you must remember is that a player can only faithfully reproduce what was contained in the original source material, so great recordings sound great and mediocre recordings can sound no better than what was recorded. A good analogy is photography, if you take a picture that is out of focus, no matter what you do to sharpen it up it will always be hampered by what was missing from it originally. The AK120 is very revealing and so you will hear tiny details that you may have never heard before.

The AK120 was great with orchestral music, jazz, rock, and anything else I could throw at it. Even better sounding in some ways than my iPod Classic 160 (playing ALAC lossless) through a CypherLabs AlgoRhythm Solo/ALO RX MKII, Cowon Z2 with ALO RX MKII and my Galaxy S4 playing FLAC files through AlgoRhythm Solo/ALO RX MKII using USB Audio Recorder Pro 1.2.7 . Now to be fair, the AlgoRhythm Solo/ALO RX MKII are an excellent combination and sound great together but carrying three components strapped together is rather cumbersome and diminished my enthusiasm. Each has its advantages but I preferred the AK to the CLAS/RX MkII combo.

Female voices like jazz singers Diana Krall, Esperanza Spalding and Norah Jones sounded superb even in 16/44 FLAC with clear, extended highs, great microdynamics and a smooth detailed midrange. There was absolutely no glassy etch or harshness, even in the high notes Esperanza hits in her song "Fall In Love" and the stand-up bass she plays through most of the album is deep, dynamic and so distinct, you can even hear her fingers sliding over the strings. Well mastered recordings just sound great and this player reveals every nuance.

Male vocals sounded great too. Among those I listened to were Jose James, Kenny Rankin and the amazing George Benson. Every essential aspect of their vocal characters came through, from Jose James smooth baritone on "Do You Feel" to Rankin's clear, pure tenor showcased in "I've Just Seen A Face" on the album, "Here in My Heart" to the amazing range and character of George Benson on "This Masquerade" whose voice and guitar sound so dynamic and lifelike he sounds as if he is right there in the room with you.

Wind instruments sounded wonderful, from the astonishing tenor sax of Eric Alexander on David Hazeltine's "The Classic Trio Meets Eric Alexander" to the agile clarinet of tenor sax titan Joe Lovano on Steve Kuhn's "Mostly Coltrane" but it was with piano music on both of those albums that the AK120 really strutted its stuff. Piano is notoriously difficult to reproduce with authentic realism and yet when playing these two well mastered 16/44 FLAC albums, the virtuosity of both David Hazeltine and Steve Kuhn stand out with the piano always crisp and crystalline in the highs, tight and without echo or rumble in the bass with natural sustained decay all of it near perfect. You can even hear the dampers being activated when the "sustain" pedal is released, you are there!
 
As far as the headphone amps I used, I tired it with the ALO RxMkII but didn't see much improvement in SQ other than a slight increase in volume and in fact, I preferred the sound directly out of the AK120. I also tried a Headstage Arrow 4G and the  JDS Objective2 Headphone Amp with similar results. The O2 was actually able to boost the volume more than the ALO Rx MkII and as it uses an alps potentiometer rather than the digital push button control that ALO uses (which raises the volume in stepped 3dB increments), it was easier to control the volume with the O2. That said, the O2 is a bit larger and therefore stays at home. For the money however, the O2 is a worthy amp to consider using with the AK and if you want to save a bit more, it is available in kit form. I initially bought the ALO amp for my JH-16 Pro and the two indeed have great synergy however, the AK120 sounds great with the JH-16's with no extra amplification needed at all and that is my out of the house set-up.

The amp I think pairs very well with both the AK120 and the HE-500 is the Lyr Headphone Amplifier which delivers a solid 4 watts per channel, enough to drive any can including the notoriously power hungry HiFiMan - HE-6 Headphones. BTW, a fantastic underrated tube for the Lyr is the Tungsram Hungary ECC85 which Schitt offers as a $50 upgrade for a matched pair instead of the GE 6BZ7 tubes. This is one of those tubes that you may enjoy so much you just won't want to switch to another to continue your tube rolling. Remember, you don't necessarily have to spend hundreds of dollars on rare, hard to find tubes to make your Lyr sound good!

I will repeat however that the AK120 sounded wonderful without any amplification when using most headphones  and depending upon the amp, may actually be  worse SQ-wise if you add one. The amp in the AK is very well designed and works well with the Wolfson DAC's, you could tell by listening to it that a lot of thought went into the design. It actually drives the HE-500 which in itself is quite a feat.