Sounds pretty similar. The HE-500s are definitely more detailed and close to natural. The HE-500s sound far more neutral and the Fidelio X1s have a more warm sound. The HE-500s are far more detailed and flat-sounding. As much as I love the X1s, I can't really give it any points above the HE-500s. Everything about HiFiMAN's outing just sounded better. The only advantages the Fidelio X1 have, really, are power consumption and price. The treble is cleaner, the bass is tighter, and the mids are far more forward. I love both headphones, but the HE-500s do far more to appeal to me.