Pros: Highly Modifiable and Organic (when paired with a great amp)
Cons: Out of the box, not the best experience
When I first got my HD650s about 5 years ago, I have to be honest and say I wasn't thrilled. Listening to them over my DAC for the first time gave me a bad first impression. I purposely loaded my vinyl transfer of The Police's "Every Breath You Take" because it has a good balance of punch and snappy low, mid and high frequencies (again, I'm talking about my vinyl transfer, not the Remastered for CD version).
I was unimpressed by the lack of "WOW Factor" that I had heard from other users/reviewers. My DAC is a high-end Crane Song Avocet which has a very good solid state headphone amp built in. I then read about how these need a few hours of burn-in time, a concept that I wasn't sure was true, as other headphones I own have sounded the way they do out of the box; this was the first headphone purchase I made where burning in was a factor.
Skeptical about this phenomenon, I decided to run pink noise while at work for two weeks straight, without listening to music during the entire burning period. After the two week burn-in period, I listened to the same song again and this time there was a noticeable difference in how wide the stereo image sounded. The low frequencies sounded more extended, but I still felt the mids and highs (especially the high frequencies) were just not as present as I've heard them on some of my other cans (Grado SR80s, Sony V6s, AKG 240DF). The other headphones don't reproduce the low frequencies as good as the 650s, so for a while I wrote them off as having a general weakness in the mids/highs.
Over the next few months, I listened to all kinds of music with them and slowly began hearing about the "foam mod" and wondered if it would make a difference. I didn't want to risk damaging them, as the smaller foam disc inside each of the drivers doesn't seem like it can go back as it's sandwiched between two layers of plastic that cover each of the drivers. I decided to do this modification, because frankly, I wasn't listening to the 650s as much as I wanted to. After removing the small piece of foam from each of the drivers, I immediately noticed an improvement in the lows and mids; the lows had less "mud" and I was able to hear more detail in things like snare drums and hi hats. The next modification was the cable, and I decided to build one using the Cardas plugs, Canare Mini Star Quad and terminate it to a 1/8" Canare plug. Being able to remove the stock plug easily through the connectors gave me the ability to see if a cable upgrade really does make a difference, and it sure does! I gave up the flexibility of the stock cable, because the heavier cable is shorter and less friendly to moving around, but I noticed immediately that the sound stage improved a bit and I started being able to pick apart elements of the audio in ways I wasn't able to before.
The only thing I still wasn't satisfied about after the foam mod and cable upgrade was the high frequencies, until I listened to these through the Schiit Valhalla. I think the tubes on the Valhalla somehow have improved the upper frequencies on these headphones. When I listen to my Grado SR80s through the Valhalla, they almost sound too bright, so maybe the amp (and stock Schiit tubes) bump up the higher frequencies, it seems like it to me, and I think this bump works extremely well for the HD650s!
It took me a few years to find a combination that has transformed these into a beautiful listening experience; out of the box, they simply don't perform to their fullest potential (they definitely are picky when it comes to an amp, and I would try them on a valve amp as opposed to a SS amp for the full experience!) I bet many Tube amp enthusiasts are having a blast with these and rolling tubes! I'm honestly perfectly happy now with these and the Valhalla.