Total noob here and I just recently got my 598's (about 2 weeks) and love them so far. They are definitely better than the EH-xxx's that I previously used. I've gone through many of the 598 threads to look for this answer, but I don't think there is a definitive answer.
From what I've read the 598's do not necessarily need an amp for them to sound good and that if there is a decent amp, the improvement will be there but it isn't a night and day difference.
I have a somewhat 'old' creative sound x-fi fatality sound card. It has a front panel for the 6.3mm plug. Is that sufficient for these cans? There is definitely a difference between the sound when I plug it in there vs the sound card from the motherboard. And is there an optimal sound level to play these at for music, games, movies etc.
Any guidance is greatly appreciated!