Music Volume Eq is a nice widget-type app (free) it givez you 5-band EQ, bass boost, and stereo-widening over any app so like you can listen to youtube tunes and it will give u nicer sound.
for a music player though you should buy PowerAmp it sounds amazing (dont bother trying the pirate versions, none of them work for long. try the free 14day trial)
you have to actually go into a music app to access the EQ, like the standard music player or Google Music, the built in EQ works great on the S3, so really no need for other apps to get for that...