I have an 12v 2250ma DC power bought from conrad. When using this external power instead of the default usb-power (from my thinkpad T420), I found that the improvement in playback quality is distinguishable to me (earphone is Etymotic ER4S and speaker is a cheap product from Hivi). More specifically, the mid range e.g. vocal is thicker with better transparency, the highs are not too bright as compared to the usb-power. Overall, the playback sound is more stable and more pleasant for listening when the external power is used.

It should be noted that this is just my own feeling, I haven't done any rigorous test. The sound quality by usb-power is already quite OK for me. However, I still think that it might be better to use a external power to reach the best performance of Babyface.