Comparison of the Oppo PM1 to the HE500 and HD650:
I finally had a chance to sit down and compare the 3 headphones - not as long a comparitive session as I would have liked, unfortunately, but enough to get a sense of the difference. I'm going to first explain the differences I heard on each track I auditioned, and then an overall summary.
I listened to the music on my Audio-GD stack - NFB 2.32 DAC and SA31SE amp. Source was Audirvana off my dedicated Mac Mini.
My bias - I like a lush, warm sound, and my reference is (where possible) the sound of the live, unamplified instrument as heard in a concert hall.
Track 1: Kazuhito Yama****a's guitar rendition of Mussorgy's Pictures (ALAC CD Rip)
The HE500s did the best job of representing the attack of each guitar note and pluck - however, the sound was weighted more towards the string, and less towards the body of the guitar, compared to the Oppos, which brought out the sound of the guitar's body a little better but at the cost of losing out a little on the "bite" of the initial pluck. The HD650s had a better sense of decay than either of the two (in terms of how the note seemed to hang in the air), but the guitar lacked a little bit of the immediacy and presence.
My preference: Oppo PM1s, but I could see how someone would like the HE500s as well
Track 2: "Quarter Chicken Dark" Yo-Yo Ma, Goat Rodeo Sessions (256kbps MP3 Mastered for iTunes)
All three did a really good job of reproducing the initial violins - the HE500s had the most presence compared to the Oppos and HD650s (no surprise that). However, when it came to the weight of the bassoon, the HD650s did the best job of reproducing what that instrument would sound like live, with its heavy notes reverbing in the chamber. The HE500 and the Oppo's bass was indeed leaner and possibly more accurate - however, real life bass is never lean (atleast in no concert I've attended), and the HD650s lusher bass gave the bassoon a very real body and impact.
My preference: HD650
Track 3: "Bolero" - Charles Munch / BSO (24/176 HDTracks)
The HE500s did not acquit themselves that well here. The snare drums are too treble-heavy and lacking in weight, and while they probably gave the most detailed rendition of the music, the music was lacking visceral impact. The HD650s and the Oppos both did a better job here - the HD650s presented the snare drums better, but the Oppos had the better overall clarity. I have to admit none of the 3 gave me goosebumps the way the LCD2s (or, rather surprisingly, the Fidelio X1s) do.
My preference: slight edge to Oppos over HD650s, although on a different day/frame of mind, i might prefer the other
Track 4: Metallica "Am I Evil", Garage Days (ALAC CD Rip)
Back when i used to do jujitsu/MMA stuff, this was the song I listened to amp myself up and even now. The initial sequence of Lars beating on the drums, and then the guitars kicking in... aaaah. Sends chills down my spine. The Oppos presented a nicely balanced sound - drums had heft, guitars were in front of the drums (as they should be) & the vocals were neither too far forwards nor overshadowed by the guitars. Compared to the Oppo's balanced sound, the HD650s were definitely skewed a little more towards the drums - they had more impact but by comparison, the guitars (starting with the riff at 1:48 or so) lacked a little bite and seemed recessed compared to the vocals and drums. The HE500s, as you might expect, were skewed the other way - the drums lacked some of the body compared to the Oppos, but the guitars were more etched. If you like a more guitar-intensive presence, you'll like the HE500s. The Oppos made me want to go mosh with my golden retriever - who was not impressed and went and hid under the desk.
My preference: Oppos (although those who like forward guitars will love the HE500s)
Track 5: "So What" Miles David (24/192 HDTracks)
The HE500s presented the music with a clarity that both the Oppos and the HD650s were missing - a lot of details, very clearly presented and with a clear, sparkling (in a good way) sound.
My preference: HE500s
Track 6: "Take Five" - Dave Brubeck (24/192 HD Tracks)
The HE500s did a very good job here - cymbals had a shimmery presence to them, and the the trumpet shone through with a lot of clarity. The HD650s had noticably more bottom end presence and bass compared to both the HE500s and the Oppos - the cymbals lacked just a little of shimmer, and the trumpet was a little smoother (normally I like smoother - in this case, I preferred the HE500's trumpet). The Oppos straddled the middle - and in this case, that didnt work to their advantage. I feel the presentation of both the HD650s and the HE500s was more interesting - both of them presented a particular interpretation of the music, while the Oppo's presentation seemed like a compromise.
My preference: Tie between HE500s/HD650s
Track 7: The Power of the Orchestra / Rene Leibowitz - "A Night on Bare Mountain"
This track has excellent dynamic range and is one of my go-to tracks for determining how much slam and impact a headphone has. The HD650s did a very, very good job here - thumping when thumps were called were for, going from pianissimo to fortissimo when needed. The Oppos, by comparison, lacked the same thumpiness and weight on the instruments, due to a slightly lower bottom end, but the better attack and initial "bite" to the sound made the instruments sound a lot more vibrant. The HE500s also lacked the rumble and thump of the HD650s, but their better treble was both their strong point and undoing - it made some portions of the music sound amazingly lifelike and timbrally rich - but it also took away a little from the impact of some parts. This one is really hard to grade, as each presentation has its own appeal
My preference: Wash (although if you put a gun to my head, I'll pick the HD650s)
Summary
The first thing that comes to mind is how good all 3 headphones sound - my preferences bias me more towards the HD650/PM1 compared to the HE500s, and the above is reflected in how often I chose those. However, the differences are relative - all these 3 headphones presented the music in a way which I found enjoyable and which i could listen to for extended periods of time.
The second thing to note is - your listening preferences are going to dictate which one you prefer. If you listen to music analytically, you'll find the the HE500s more towards your taste. If your goal is a rich, full-bodied sound with lifelike bass (not to be confused with "lean and accurate bass" - there is no such thing. Think bass that shakes the paint off the walls and hangs around in the room). The Oppos are technically better than either of the two, IMO. Both the HD650 and HE500 have a voicing bias, whereas the Oppo - while also warm and smoothly voiced - is the most balanced of the lot. That can be a good thing or a bad thing, depending on your preferences. If you have a strong preference for PRaT or for a clear, airy sound, you'll prefer one of the other. I personally wouldnt be able to settle on just 1.
If the HD650s had a little more bite in the high end, or the Oppos had a little heft in the bottom end, that would be a different matter. IMO, the Oppos do better across a broader range of music - but where the HD650s excel, they REALLY excel.
The most surprising thing to me was how good the HD650s still are, and how well they hold their own against newer headphones. I was planning on selling the HD650s now that I have gotten the Oppos, but now I am actually considering selling the HE500s and keeping the HD650s. That being said, to be fair to the HE500s, they arent really voiced according to my listening preferences, so my own preferences are having a big impact on my evaluation.
Hope this helps.