I've seen this question, and I've been dreading it. Because it is a hard question to answer. I wanted to make sure I had time to write a full comparison to not slight either headphone. I will preface the comparison by stating I enjoy the sound of open cans. It's not just because they produce a better sound stage. It is because of a ethereal quality the sound possesses. I also want to bring into the discussion my DT990 Pros. I own the 3 full-sized headphones I do b/c they all serve a purpose.
The HD650s strength is its warmth. Definitely a lower half of the spectrum headphone. The sound stage is amazing, but you are a few rows back. Majority of the sound plays in front of you. There is a slight roll off at the high end, but that is okay...because the HD650 never tries to be a headphone that is "balanced". It is a great phone to sit back and enjoy the music. When I want to relax, these are the headphones I always reach for. And like many have said before me, they scale beautifully. The more juice I give it, the more enjoyment it gives me. They leak, so I use them for home listening. But they are very dynamic and good for all genres of music. I especially enjoy them with classic rock and classical music. Pop a bottle of pinot noir and zone out for a couple of hours. If these cans have a weakness, it is that roll-off at the highs...aka the veil of Sennheiser.
The DT990s are my analytical pair. There is a pronounced V shape sound, but I find when amped by my yamaha receiver, the lower mids REALLY fill in. But the extreme clarity of the highs is ever-present. Its a bit exhausting to listen to these cans. They are always surprising me with something I never heard in a song. I'm always engaged and jubilant when I listen with these. These are great for acoustic guitar and pop. Whereas the Senns roll off the top, these shine with crisp clarity. And whereas the Senns bass is warm and flowing, the DT990s are even analytical in the bass. There is great separation from note to note, so it is fantastic for listening to an artist like John Entwistle of The Who fly his fingers up and down the scale. Won't have the bass impact of Senns, but the separation brings a different enjoyment at the low-end. They are pretty much the exact opposite of the HD650s. These cans' weakness is two-fold. Some may find the highs a bit too piercing and the mids a bit recessed.
Finally, the K550s. These are the most balanced pair of the bunch. Oddly enough, these, although closed-back, have the least bass of the 3. Seems kind of counter-intuitive, eh? The sound stage isn't as vast as the other 2, but it is amazing for a closed headphone. In fact, its sound stage is bigger than that of a lot of open-back headphones. These cans have decent bass, but very little to no sub-bass. These required the most burn-in of the 3. These cans roll-off the top, but not as much as the Senns. It's not a complete flat sound signature. I would call it a mesa-shape. There is a dip at the very low end, and another dip at the very high end. These cans are a jack-of-all-trades, and a master of none. Do they seal? check. Do they have bass? check. Do they have a decent mid? check. Do they have clear highs? check. The only real weakness is with electric guitar and music with a lot of electronic distortion. When there is a lot of electric guitar, like bands like Radiohead and Linkin Park (two of my favorites), these headphones struggle. There is a distorted, garbled, gurgle when trying to portray heavy electric guitar, and electronic distortion. I find these VERY bad for hard rock/heavy metal. And electronic music with a ton of distortion, like Moby per se, these cans struggle in this same manner. These shine when playing pop, techno, classical/opera, and acoustics. I own these for one reason...because they have amazing sound stage with a closed headphone. I will hardly ever pick these up when i'm at home. They are my good traveling pair.
So in a long-winded answer...i can't compare the two...let alone three. They are all so different, and I use them so differently. These are all 3 great headphones, and they all reward me in vastly different ways. Synergy in my cans is what I've achieved by owning the 3.
