The 58X has more sub-bass than the 660S and frequency-response wise the 58X sounds close to the 660S, it belongs in the new generation of 600 series headphones, they are more lively and easier to drive. But I think the 58X is less refined, at least I want to believe in this, because it wouldn't make sense otherwise, maybe the 660S has better coils or something (these details). Comparing the 58X and 660 I read people say that the 660 doesn't have that grain, the 660S is more detailed and resolving as well (whatever that means). But in general the 58X is close to the 660, it's like a rough diamond. Well I can say for me the 58X has some grain, something in the treble region that can bother slightly, after some time cause fatigue, but after time passed and using EQ it got better and a lot of people don't have this issue at all. Also heard that the felt mod helps with this grain but I never tried. But for $150 the 58X is great, the grain isn't bad at all, you should get it.