Currently I have one I9100 and I don't even think try to amplify that crappy output signal. Nothing good can come out there. Worse sound quality in a cell phone ever. My 5 years old Nokia N95 8Gb or my 2 years old Nokia N97 sound way better straight out of the jack
If you don't mind that derelict quality of the Sammie, you can try Cyanogenmod 7 with the DSP and PowerAmp Pro player, but doesn't get much (or any) better, tbh.