Never really enjoyed the Hifiman He 500 that much. The soundstage was very closed and they were heavy and cumbersome. Don't even start me on that thin silver/copper cable. Cant compare He500 to the HD800 which is in a class of it own. Hifiman needs to work on quality control issues. Maybe one day they can pay as much attention to detail as Sennheiser pays to the HD800. Especially with each headphone getting its frequency graph. Just look at the fine mesh that goes over the driver on HD800, means nothing was left to chance. You dont need to worry about dust/hair getting in the driver, which means perfect sound every time.
The Hifiman definitely has some quality control issues. But at less than half the cost of the HD800, I can forgive some cut corners. I replaced the crappy cable it came with as I couldn't stand its microphonics. I doubt Hifiman will ever be up to the manufacturing standards of Sennheiser, but Sennheiser is also a much larger company with more resources than Hifiman. There's definitely room for improvement though. I love both headphones but overall I prefer the sound of the Hifiman. Much more euphonic and fun to listen to, even if it is technically inferior. Also, the Hifiman has dust screens on both sides of the driver as well.