I hear you eso6686! I think many people share your sentiments.
I'm a huge Sony fan, but their NWZ-ZX1, which no doubt has a great UI, only offers 15mW into a 16-Ohm load - too weak for anything except IEMs and the most efficient of headphones. With the right transducers, the ZX1 might make an awesome DAP.
But worse, Sony designed their PHA-2 portable DAC/amp, with the convenience of USB charging (5V), which limits them to using a single-cell, 3.7V lithium-ion battery, the voltage of which probably has some role to play in limiting the output power of the PHA-2's headphone amp to only 58mW into a 50-Ohm load - per their specs.
Ironically, the PHA-2 uses a chip amp, called the Texas Instruments TPA6120, that's capable of outputting 1500 mW into 32-Ohm (or 960 mW into 50-Ohm) when given a 12V supply voltage, and a whopping 2000 mW into 32-Ohm (or 1280 mW into 50-Ohm), with a 15V supply voltage! See Figure 8, on page 8 of the TPA6120A2 data sheet:
http://www.ti.com/lit/ds/symlink/tpa6120a2.pdf
But the PHA-2's TPA6120 isn't putting out 2000 mW into 32-Ohms. That chip needs 12V for 1500 mW or 15V for 2000 mW (into 32-Ohms). The PHA-2 is using a one-cell, 3.7V battery. ?????
The iBasso PB2 portable headphone amp uses a 3-cell, 11.1V battery (with different op-amps, of course). The RSA SR71B uses a four-cell, 14.8V battery (with different op-amps). The HiFiMan HM-801 and HM-901 DAPs use an 18V serial array of two 9V batteries to power their amp sections - that's why they run so hot. Triad Audio's L3 offers an external battery option that can supply up to 30V to the rails. The correlation is obvious: Weak battery = weak power output.
So, even though the Sony PHA-2 portable DAC/amp allows something akin to an I2S digital connection from the ZX1, after spending about $750 for the ZX1 and another $500 for the PHA-2, you'll still be looking for efficient headphones -or- you'll be looking for an external amp that can be connected to the PHA-2's Line Out jack!
Unfortunately, Sony is (smartly?) going after the much larger community of buyers who demand small devices with long play times. It kills me that the PHA-2 contains a TI chip capable of producing 2000 mW into 32 Ohms (with a 15V supply voltage), but Sony gives us no way to connect anything but a 5V external supply! They could have satisfied both markets (with sufficient heat sinks...).
Arrrrgh!
Mike