After other readings, I add some more considerations about the compatibility of a smartphone with our E18.
I read better a post in this thread from DanBa that link this other thread on head-fi and I think that I have understand new things.
In the list of stock Android-powered devices reportedly interworking with standard USB DAC some phone can work with a DAC with stock roms wich means that the software got USB OTG feature AND USB Audio driver.
Some other device canr work only with USB Audio Recorder Pro wich indicates that it does have proper hardware for USB OTG but not USB Audio support.
In this list the Galaxy Nexus is reported to work with USB AUdio Recorder Pro, so, if I have understand, this means that the Galaxy Nexus have NO hardware limitation, but only software.
Later I make a test with the trial of UARP on my Gnex.
Prehaps, since the Galaxy Nexus is very close to the Galaxy S2, this last one can support the USB OTG, but I don't found the S2 in the above list, so I don't think so (if DanBa followed the linked thread and would like to add something, it would be really appreciated).
So the question is: why I must use an App that is intended for record audio (is not a real player!) only to add usb audio? Is so hard to introduce this feature in Android? Is possible for a developer create an app (I pay for it!) that add only this feature and allow to use the favorite player in Android?