As I said, the HE-500 is an upgrade to the HD600 in terms of technicalities..the HE-500 is faster, clearer, more dynamic, better extended, and possess a larger soundstage. They share a great deal as far as sound signature, too (probably the closest I've heard to the HD600). If someone is looking to hear an advancement on the HD600 sound.. the HE-500 would be a very good choice, IME.
To my ears, the HD600 sounds more laid back & smoother sounding than the HE-500. While the HE-500's gentle bass boost was tuned in a
very tasteful manner.. I came to find it slightly too thick for my taste. The HE-500's midrage, while impeccably smooth and a showstealer in terms of clarity and detail, sounded slightly more forward than I desired. It's certainly not Grado-esque in aggression but the forward nature of the midrange was easily identifiable to my ears. The dip in the upper midrange & lower treble (around 4K) gave the midrange a slightly darker tone than I had hoped for.
The HD600 has a slightly more prominent midbass boost than the HE-500 but the HD600's midbass has a more airy, open quality to it vs the HE-500 bass' thicker quality. What's important to note though is that despite the HE-500's thicker bass response.. it's still incredibly fast, effortless, clear, and clean (thanks to the excellent ortho driver). My issue was really with the HE-500's bass texturing more than some technical shortcoming (cause for the price, there really isn't any). Like the HE-500, the HD600 midrange is positioned slightly forward in the mix..but the HD600's midrange doesn't feel or sound as aggressive as the HE-500's midrange did. Both the HD600 & HE-500 have smooth, gently rolled off treble..the HD600 feels like it's got a touch more air and openness while the HE-500's treble is buttery smooth and clear.
For me, the decision came down to personal taste more than anything. The airy, laid back, romantic feel of the HD-600 was simply too seductive