I already answered this and would rather not compare too much. X5 is more powerful and fuller sounding, ZX1 is colder and airier with wider soundstage. The ZX1 UI is better, the X5 sound is more versatile but the ZX1 can outshine it on certain pairing. So on par but very different SQ wise and edge to the ZX1 for UI and design. Of course X5 is half the price.
I'll add that the ZX1 only outputs 15mW into a 16-Ohm load - weaker than an iPhone - so I wouldn't recommend it for use with anything but IEMs and very efficient headphones.
And don't bother with connecting the ZX-1 to Sony's PHA-2 DAC/amp, with anything but efficient headphones. Even the PHA-2's output is only 58mW into 50 Ohms.
I don't know what the specs are for the X5 output power, but it would really surprise me if it's less than 58mW into 50 Ohms - that's weak! And 15mW into 16 Ohms is even weaker.
I think Sony sacrificed power for play time in both the ZX-1 and the PHA-2. But they probably sound pretty good with transducers that can get by on very little power.