Hi all --- I cannot speak to the Galaxy SIII --- but I have been testing extensively with the Nokia 808 which has USB OTG (along with its ancestor the N8.).
I can confirm:
1) Fiio E7 and Ordnance.25 work perfectly -- they also have switches to turn off USB charging, thus preserving battery on your phone
2) The RSA Predator works as well -- but you can not turn of USB charging so it will drain your phone battery by 10% to 15% per hour.
3) NONE of the AMP/DAC offerings from Pico, Ibasso, or GoVibe work. Neither does the Fiio E17. (I have tried them all :>) )
I am not particularly technical but the following conditions seem to be barriers for not working.
1) Excessive Power Consumption when connected to USB --- It seems like the predator draws less power than the Nokia 808s cut off limit so you will get an error warning but it will still play. The phone will refuse to recognize other more power hungry DAC/Amps.
2) There seems to be issues with 24/96 DACs -- this is only a hypothesis -- there could always be an exception.
3) Devices that automatically try to install drivers seem to cause connection errors on USB-OTG
I guess this supports some previous posts but I thought I would share my experience here.