Put the lot on a speaker amp with a quality power supply and the 6 takes it IMO. I had both for a direct comparison and I kept the 6. I'd say the 500s gets 90-95% of the 6s being pushed hard. The sound at low levels were pretty even. I liked the extension detail, more open soundstage and just a bit more resolution.
If you put all of the headphones discussed recently on speaker amps then some of them would be over- powered, so in that sense I'm sure the HE6 would win.