Pros - Supports iDevices, Android 4.1+, and PC Audio. Manufacturer configurable
Cons - Non-intuitive user interface. Integrated DAC/Amp, therefore can't mix 'n match. Default base output impedance of 47 ohm
This review is somewhat different from my other reviews as my Go-DAP X was personalised to my requirement. This service is offered by VentureCraft of which many other Go-DAP X owners have taken advantage of. Therefore this review may not necessarily be completely applicable to the base Go-DAP X model.
The Go-DAP X was released last year around early October 2012 & was the somewhat more universal version of the Go-DAP 4.0 which was iPhone 4/4S specific.
Back then, the Go-DAP X was the first portable DAC/Amp that officially supported not only Apple's iDevices but also Samsung Galaxy S3's (and subsequently Note's) iteration of Android. And as such it also supported USB Audio for PCs, & notebooks.
The X actually sports two DACs, AKM AK4353 & PCM2902C Stereo Audio Codec with USB interface. The AKM is used for iDevices whilst the PCM2902C for USB Audio (Android 4.1+ Jelly Bean & PC Audio).
My particular Go-DAP X's amp was customized by VentureCraft. Instead of the base offerings of OPA2134 or Muses8920, both with output impedance of 47 ohm, mine was customed to use the LME39860 OpAmps & headphone impedance out to 10 ohm. This by default also meant my power output per channel jumped from 80mW/ch to 130mW/ch (for iDevices & 150mW/ch for USB Audio).
The Go-DAP X also sports a 2200mAh battery which can be used to charge the iPhone during play mode. It could also be used to charge Android phones (or other USB devices) but would be in battery-mode only during such purposes.
There's is also a bass boost option for iDevice transports, and a 3D surround sound simulation.
As with the Go-DAP 4.0, there's an optical out to use the Go-DAP X as a DDC. The optical output seems to lock to 48kHz sampling rate.
As I use the X primarily with my iPhone, the following applies to the AKM AK4353 DAC. This DAC seems to have proven itself as it has been used by VentureCraft for their past Go-DAP 4.0, the Go-DAP X, as well as their Go-DAP DD. The DAC is able to retrieve quite a decent amount of detail but present it in a subtle form instead of flat "wham-in-your-face" impact (which may grab one's attention at first but then loses the ability to present a depth imaging perception nicely). Soundstage is also quite nicely presented with sufficient width.
The original amp in my X was the base OPA2134. Possibly it wasn't burnt in well enough but it sounded somewhat grainy. The LME49860 OpAmp option was much smoother to my ears and to my preference. Possibly (most likely) due to the 47 ohm to 10 ohm mod, the bass of the LME49860 also extended deeper with a smooth layered rendering. The mids are lush although possibly somewhat less detail than dedicate multi-component DAC/Amp stacks. Meanwhile the trebles are airy but not bright.
I particularly enjoy pairing the X with my FitEar MH335DW which is known to be a warm & bassy IEM. The 10 ohm output impedance actually tapers off the bass but still in a controlled manner. I've tried other amps with a 22 ohm output, or the original X with 47 ohm and the roll off with those setups were too much but with the X it was perfect to my ears.
On the other hand, if a user has a somewhat more neutral ear/headphone and seeks a little more bass, there is a bass boost option which will fill in the bass very nicely and naturally. I'd like to stress that at least to me (who generally doesn't use EQ or bass/treble boost), this bass boost isn't gimmicky to me. It adds subtly without sounding artificial.
As a portable package that can be used with not only iDevices but with Android Jelly Bean (4.1+) and PC Audio, not to mention it can be used as a portable battery for iPhones and other small USB-charged devices, it's hard to fault the Go-DAP X. Naturally the sonics cannot be compared to other dedicated multi-component stacks (which cost a lot more and are larger and less portable), however it's a neat portable integrated DAC/Amp for those who want a step up from audio straight out of the portable phone.
Edit: VentureCraft actually seem to have the Go-DAP X with LME49860 by default now as their base OPA2134 stock seems to be running low. However their LME49860 versions are still at 47 ohm. Therefore one has to explicitly request them to mod OI down to 10 ohm.