I found that the Touch 5G had the best USB Audio implementation. Its internal DAC and Amp are superior to the other devices when used standalone or with an external Amp (double amping). Once connected to an external DAC the differences using the same software with the same DAC/Amp combinations were minimal. IMO Neutron MP was the best. gMusic2 (plays Google Cloud and All Access) was better than Google's Player software in both SQ and UI.
I found all of the Android devices screamed for help as their internal Amps were nothing to write home about. Since they don't have line outs (or any good line outs) the quality of their DACs are hard to evaluate since they pass through unimpressive amps. The GS3 is capable of line out via a special USB cable, unfortunately it's noisey.
I much prefer Android to iOS, but for a DAP only use, this is not an issue. Although Apple has much better hardware for the Touch 5G as well as a better USB Audio implementation, based upon conversations that I've had with someone that writes DAP software on iOS, they did not do a good job of allowing the sharing music files between applications. But, it can be done. Until this is done, locally downloaded music files from Google using either Google Play Music or gMusic2 are duplicated in different areas, wasting space on the SSD.
I don't quite understand your logic.
MP3 aside, can you please specify what types of music files did you try to play? I.e. did you test higher resolution music files with BD/SR above 16bit/44.1kHz?
And finally, why do you conclude that Touch 5G has the best USB Audio implementation by claiming that its internal DAC/Amp is the best?
I don't argue this statement about internal DAC/Amp, all I am saying that the internal DAC/Amp have absolutely nothing to do with USB audio implementation, as they are totally bypassed...
And then, we're left with all the stupid Apple things like no SD card support, iTunes nightmare to put music into the device, etc.
There have to be very solid reasons to claim iPod has the best USB Audio implementation... Oh, and can you please also tell us what current allowance does the iPod touch have, when you connect an external device to it?