I have Sony Xperia ZL and Sony external DAC/Amp PHA-2 connected via USB in OTG mode. Combination bypasses internal DAC and produces great sound via USB Audio Player Pro but USBAPP is low function. Xperia ZL is one of many Android phones that won't allow audio to bypass internal DAC unless custom software interface (USBAPP) is used. While it is true that just-announced Android phones are claiming internal audio bypass as a feature, there is a significant existing base of phones that can use USB in OTG mode with proper drivers. Foobar 2000 Mobile should allow use of external DACs via USB in OTG mode. (My rig is available for testing when/if that point arrives.)