Thanks to Justin at Headamp for letting me demo the Hifiman HE-500 headphone.
I A/B'ed the two headphones for about 90 minutes. I used an XRCD Ernie Watts, "Classic Moods" jazz cd and a Roku Soundbridge internet radio. I only listened to jazz, which is my main genre I listen to. I should point out that my HD-800 is balanced vs. the HE-500 being single-ended.
Bass: The HD-800 has about the same extension as the HE-500. Both headphones could use a little more extension. The HD-800 bass is a little more tighter, but the HE-500 is not boomy or muddy. The HE-500 has more mid-bass, which can give the impression as having more bass. I prefer the bass quality on the HD-800.
Mid-range: The HE-500 has a more forward mid-range. It's almost Grado like. It's very smooth. I prefer the mid-range to be neither forward or recessed, therefore I prefer the HD-800 mid-range.
Treble: Both headphones have great treble extension. The HD-800 may have slightly more extension. But the HE-500 treble is very smooth. The HD-800 can sometimes be just a little bit sibilant. I prefer the quantity and quality a little bit more on the HD-800.
The HD-800 is very detailed. You can hear every little detail. The HE-500 is not bad in this regard, it's just that the HD-800 is so good. The same can be said about the soundstage. This is where HD-800 excels. It has an expansive soundstage. The HE-500 is good, but no where near the separation the HD-800 has. The HD-800 is a very open sounding headphone. The HE-500 does not have the openess that the HD-800 has. I prefer the HD-800 detail, soundstage and openess.
The HE-500 is a fairly heavy headphone compared to the HD-800. The HE-500 is pretty comfortable considering it's weight. The pads are very comfortable. I prefer the HD-800 because it's lighter and the pads are even more comfortable than the HE-500.
For me listening to jazz, I prefer the HD-800. The HE-500 is a great headphone at $700. I could be happy with it. But when A/B' ing the two headphones, I have to go with the HD-800. The more difficult question is the HD-800 worth $1500 vs. the HE-500 at $700 to you. For me, it's the point of diminishing returns. But for me, it is worth it. The HD-800 is an awesome technical headphone for the jazz lover. I can't speak about the HD-800 vs. HE-500 for other music styles. I can say though that you would be happy with either headphone at it's price points.