Pros: Fantastic sound, mod-able
Cons: Weight, cable connectors, stock cable, stock pads
The HE500 brings one word to my mind: Immersion. I've never felt closer to the music. Period.
It's the most organic, musical headphone I've heard to date. This implies a preference over the D2k, D5k, RS1i, GS1k, HE400i, ZMF Master, T90, HD600 and HD650. Just a home-run of an effort from Hifiman that, aside from a few aesthetic/comfort issues, makes these an end-game can.
Test Songs (all ALAC either 16/44 or 24/96):
Someone Like You - Adele - Live from Royal Albert Hall
Late in the Evening - Paul Simon - The Essential Paul Simon
Mahler Symphony 5 - Georg Solti & CSO
What About Me - Snarky Puppy - We Like It Here
Hide and Seek - Imogen Heap - Speak for Yourself
Prelude from Cello Suite #4 - Yo Yo Ma
Hot for Teacher - 1984 - Van Halen
MacBook Pro > Fidelia > Pan Am Stack
Well, in a word: real. I got my first taste of planar bass early last year with the HE400 and even though it punched more and lower than my flavor of choice, the RS1i, it didn't stick with me. I've got the 500s now and they have the most realistic bass I've ever heard from a headphone. The sub-bass is present, the mid bass is dynamic, and the upper bass/lower mids are nice and clear and textured. The range of Ma's cello can be heard with ease (something not easily done!). Kick drums on the Snarky Puppy and Van Halen tunes don't sound like shapeless blobs. They sound distinct and punchy. This beats any bass I've heard from a dynamic hands down. Sorry D5000 and HD650!
Rich! Neither forward nor recessed. I don't think I'd say that the HE500 is a mid-centric phone, but in my experience the mids are often the defining characteristic for most upper-end headphones. That's where vocals, guitars, and horns predominantly sound. Lots of guys can live with hot treble or sloppy bass, but if the mids aren't palatable, whether they say or it not, they aren't satisfied. The mids are very lifelike in they're portrayal of texture and detail. The horns in the Simon and Snarky Puppy are very clear and textured. Vocals sound excellent in "Hide and Seek." Very organic and rich. Once again, my hat is off to Hifiman. I thought I'd never find electric guitar I liked better than the RS1i. Even though I'd love to have both, the HE500 presents vocals beautifully AND I get the crunch from my favorite guitar riffs. Truly an accomplishment.
Natural and clear. The HE500's upper end is quite special. While not unnaturally-detailed (what I think of when I hear the HD800), the treble range is still very detailed and revealing. It bests anything I've heard from Beyer and certainly Grado. Intricate cymbal work on "Hot for Teacher," violin overtones in Mahler, and air in Imogen Heap's voice all come through clearly and naturally. Probably the only time I've been able to say that about any headphone. A comparison I've often heard is that of the HE500 to the LCD2. The treble has to be the biggest difference. If you want polite, smooth treble, go with the Audeze. However, if you want real treble, go with the Hifiman!
The soundstage is excellent. Once again, just real! Not overly huge a la Denon, or pre-shrunk like a Grado. Studio recorded tracks have excellent space and imaging. Live albums sound live! All instruments from orchestras to jazz bands to duos can be picked out with ease. To me the sound sits just in front of "midway" around the head, but once again, nothing sounds fake or doctored.
*Note: I have the new Focus Pads on my 500s. These are slightly angled in an effort to improve sense of space. I hate the stock velours and pleathers so much that I won't offer an A/B but I will say that the Focus Pads are much more comfortable and certainly don't take away from anything that was already happening. If there is an improvement in sound over the stock velour, it's minimal to these ears.
The only qualms I have with Hifiman's hit are those in the categories of comfort and accessories. First, I have gotten used to it, but when I first gave the HE500s a go, I thought the weight would be a deal-breaker. Remember, I'm a recovering Grado-holic, so I'm quite used to wearing headphones that weigh next to nothing. Luckily the headband fits my head quite well (like a Grado) and I've switched to the Focus pads, which are softer, and distribute weight more evenly. I still am reminded of the heft of these guys from time to time, but it's certainly not unbearable. Second, the stock cable and pads are well, just awful. The stock silver cable is the most microphonic cable I've ever heard. Worse than any of the 75+ IEMs I've tried. Since most of my listening is done at the desktop, this doesn't come into play too much, but for a premium headphone like the 500, this is to me, a ridiculous shortcoming. Also, the pads. Pleather: 2 thumbs down! Velour: 1.5 thumbs down! Neither are comfortable, both are hot, the pleather muddies up the sound, and the velour feels like sandpaper. Once again, silly issues to have for a headphone that is so awesome!
If you couldn't tell already, I'm a huge fan of the HE500, even though for me to really love them I had to buy some new pads and switch up the cable. They offer the best sound I've ever heard. And by best, I mean most lifelike. No overly hyped-treble a la upper-end Senns or Grado. No head-shaking bass a la Audeze. Instead a presentation that is very complete, organic, and most of all fun! These are my thoughts and I'm sticking with 'em!