After much deliberating, I decided to get these instead of the more expensive 660s. Since there's still heated disagreement over which is better, spending an extra $150-200 didn't seem like a great plan. My old, but still functional, phones are the oddball KRK KNS8400.
So far, I'm having mixed results. They sound good--very good. I see no sign of the mythical "veil" no matter what I plug them into. They just don't quite have the night-and-day improvement I was hoping for, I guess. Best-case, they seem about 50% better than my old ones, less than that when using the Sonarworks equalizer to flatten the KNS8400's weird frequency distribution. The software has much less effect on the HD650, since it's already so flat; it mainly just bumps the sub-bass up a bit.
Instrument separation (I hesitate to say "soundstage" because neither headphone feels very wide, especially compared to an HD579 I tried in a store) is the most obvious improvement--every source of sound feels distinct and easy to pinpoint on the HD650, while on the KNS8400 everything is more squished together. Maybe just because one is open-back and the other is closed. Texture, timbre, detail, or whatever you want to call it is also better with the 650; the difference isn't always noticeable, but individual instruments (or synth effects, or whatever) sound better, more defined. I wouldn't say there's more detail in general; I notice things I hadn't before in some songs, but going back to the KNS8400 those components are (usually) still audible, just less obvious. Though, maybe that's what people really mean when they say a headphone is more detailed? "Sparkly" effects seem a bit faster and clearer on the HD650... so yeah, definitely not "veiled." I can kinda hear where that idea comes from by using the equalizer to simulate a more treble-heavy headphone, but doing so makes the overall sound feel unrealistic and a bit grating.
I got a Magni 3 to play with since everyone insists these 300-ohm phones need amplification. At the moment, I'm classifying it as a cute toy that sounds exactly as good as my motherboard's Realtek chip. Louder, of course, but not better, and I was getting more than enough volume straight from my laptop (25% of maximum, with Sonarworks reducing the levels even further). Messed around with low/high gain, maxing out Windows volume so it's "bit-perfect" or whatever, didn't seem to change anything, other than loudness. I don't have an external DAC because I'm even more skeptical about those things. I'm sure all that is blasphemy to many of you, but I really can't hear a difference... though if anyone knows of something more specific to listen for, I could give that a try. The main problem may just be that I listen at extremely low volumes compared to most people, so I have no need for ultra-low distortion at painful loudness levels, or anything of that nature.
Still, I had a similar underwhelmed reaction after getting my KNS8400 a few years ago. Before my HD650 got here, I dug out my ancient Sennheiser eh350 (which I used to think were quite good) to compare with the 8400, and they sounded terrible. Similar to the Momentum 2.0 I found in a store, just really grainy and... well... veiled. So maybe I just need to get used to them. Or maybe break-in is a thing.
Footnote: The infamous "clamping" is there, but it doesn't seem that bad to me. The KNS8400 was much worse when I first got it.