Thanks for the great review. I am also looking to upgrade my HD600 that I've been using for the past 11 years. So far, I've tried HD800, HD700, HE-500, and I'll be auditioning the LCD-2.2's tomorrow. I mostly listen to rock and pop. I use a souped up PPA amp. A couple of different observations:
1. I don't agree with the Lamborghini analogy for the built quality of Hifiman. To me, it feels like a well executed kitcar at best. HE-500 is not all metal. Cups are some sort of plastic or resin. Cable is disappointing, in that if you bend it, it stays bent (silver ones). I'd expect some more flexibility at this price level. Pads are hard; a lot more so than the HD600 pads. Connector is a cheap molded plastic. On the other hand, they used a thin but effective memory foam padding on the headband, which is very comfortable. The headband is covered with what appears to be genuine leather. So I think that Hifiman's effort in build quality is clear, but overall it still looks handmade, meaning that some parts look aftermarket or cheap. This is in stark contrast with say HD800.
2. It's heavy, pads are hard, cable is not very felxible. But somehow I can comfortable listen to it for a long time. Of course, HD600 is clearly more comfortable.
3. Soundwise, Hifiman offers much better bass, less mid-high's, much better clarity, detail and air. HD600 definitely hits every note, but everything feels "compressed" compared to the HE-500. Haveing used HD600 for 11 years, I am an avid fan; but I find HE-500 more enjoyable; even more so than the HD800, which is superior in many ways to both headphones (soundstage, detail, air, etc.)
I won't sell my HD600's no matter what I buy next. Given that, I am not sure if I'd pay $700 for a HE-500. It's a tough decision. But if I were buying new, I think I'd go for the HE-500.
Of the ones I auditioned, HD700 was a clear disappointment though. It offers better bass, more clarity, but balance was way of. Weird. No wonder the price dropped from $1K to $650.