This is the crux of the matter, the most important part. Before going in, I have to mention that I am not one to use audiophile type DAPs. I like my smartphone (Nexus 5) paired with an exterior AMP (Fiio E12). I have become familiar with Cyanogenmod as well as the ability to tailor the sound with various apps, and I haven't found a reason to make the leap yet.
I had the chance to try several portable players including Fiio's X5, and Colorfly's C3 a while back. Sadly, I don't have X1's bigger brother in hand at the moment, so my comparison is going to focus on Colorfly's C3, which is another budget DAP, albeit a much older one.
X1 features a neutral and warm sound signature. It features decent instrumental separation and soundstage, though there's room for improvement in this department. It doesn't end up feeling dull or muddy, still retaining an impressive amount of clarity and detail for its price-point.
The bass has full presence and sounds full bodied, managing to keep up with punchy beats with ease. On the other hand, the mid-range is the most proeminent, warm, full sounding and retrieving a good amount of detail. In fact, X1 behaves exceptionally well with vocals. Lastly, there's the soft rolling treble that could do a lot more if it had just a twinge more sparkle.
As one might expect, this is not as powerful as the X3 or X5, its two older siblings. It can drive most IEMs with no loss in quality, but I found that with headphones with impedances over 60ohms, X1 needs some amping to properly drive them. I paired it with a number of headphones including BrainWavz S0 and S1, Sennheiser Amperior, Sony MDR1-A, and had no issues with them. On the other hand, when I used BrainWavz HM5 and Sennheiser HD600, there was a loss of noticeable loss of dynamics compared to playback using an exterior amp, like the E12.
Fiio rates the Fiio X1 at about 12 hours of playback, 15 with the firmware update. Well, in this department, the rattins is pretty accurate. The 1700mAh battery keeps the DAP running for about 11 hours continuously. Charging is also relatively quick, about 3 to 4 hours, and you can still play media while the player is charging.