I'm not sure if it behaves differently on an i9300 since it is technically different hardware (diff CPU and DAC) than the i747m. The 4.3 update also moved that setting, it's now: Settings> My Device> Accessory> Audio output mode. And yes that setting needs to be TICKED because when I unticked it, audio came out from the onboard speaker and I had to unplug and replug the E17 to get sound through it again. Audio output is also limited to 16/44 with native player and PowerAmp. You need to use UARP to get 24/96 output through USB OTG. I have no clue whether the E18 will work with a GS3 with 4.3 and I think it would be best to ask in the E18 thread. From what I recall, Fiio was having compatibility issues with the different Galaxy phones depending on Android version during their development. They list the GS3 as compatible, but I suspect that testing was done with 4.1.2 and not 4.3 because it was so recently released. And jumping from 4.1.2 to 4.3 made a previously non-working E17 work for me. The reverse could also be true for the E18, but I don't have any data confirm or deny that. Best bet is to check that other thread or email Fiio and JDS Labs directly to confirm before buying.
Just finished updating my Nexus 7 2013 to 4.4.2, no change in behavior from before, updated my previous post here:
http://www.head-fi.org/t/595071/android-phones-and-usb-dacs/3615#post_10046022