I've purchase both the HD58X and HE4XX in the last few months. I like them both but each shine with different music.
I've never owned the 6xx series, but did have the HD700 for a short time and didn't care for it.
While they don't need an amp, I would STRONGLY still consider one. I was pretty underwhelmed using them out of my DAC as well as portable devices. I have a MusicHall ph25.2 headphone amp and it takes them up a few notches.
The HD58x are like putting on your favorite sweatshirt, warm soft and comforting. The HE4xx are like getting ready for a night out on the town.