USB power alone is actually fine if a decent power step-up circuit is included. The problem with Scarlett 2i2's headphone-out goes a bit further than just lacking a good power supply. The opamp that is used for the headphone-out is not actually suitable for driving headphone. To make it work, Focusrite has to put in resistors and capacitors in the signal path to make it works, but those also serve to degrade SQ. To put it simple, the headphone-out is really more in line of 'making sound; rather than 'making good sound'.