I'm considering getting a pair of Sennheisers, I see HD650 is still available in some shops, but I have not yet decided on getting HD600, HD650 or HD660S, they are equally priced depending of the different retailer.
Here are some thoughts:
https://www.reddit.com/r/headphones/comments/7w8htj/_/dtyeqie
https://www.soundboxlab.com/head-phone/over-ear-headphone/hd600-vs-hd650-vs-hd660s/
I listen to a wide variety of music: piano, classical, electronic, classic rock.
For what it's worth, Sennheiser's view on their product line seems to have shifted over time. The 660 was meant to replace the 650, and the 650 was meant to replace the 600, but even though each iteration was intended to fix the previous model, they all introduced new issues that alienated a certain portion of the fanbase (see Tyll's legendary 660 review for a good example). That said, they've all continued to sell pretty well, and as far as I understand, Sennheiser plans to keep all three models in production at this point (they recently revamped the aesthetics of the 600 and 650, so it seems unlikely they'll get discontinued soon). Depending on your preferences, all of the Sennheiser 6 series are viable contenders. They all share the same general sonic profile--all are a somewhat warm take on "neutral" with an emphasis on the mids, all have an intimate soundstage (not particularly wide for an openback), and they generally have no real sub-bass and slightly muted treble--but each model has some slight differences:
main Sennheiser line:
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600: less bass than the others, slightly shouty upper mids, not especially bright but beloved for their clear treble response (slightly less even treble than some of the others but it's known to sound quite pleasant and has more focus on the treble details). 300 ohms. In the US, typically costs $300-350. short version: generally beloved for its treble response.
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650: slightly warmer / darker iteration of the 600. more *upper bass, warmer low-mids, still a touch of upper mid shout but less than the 600, and the treble is smoother overall than the 600. 300 ohms. In the US, typically costs $350-400 (but see the 6xx below). short version: known for lush mids and smooth presentation overall.
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660: presented as a slightly more upscale model (and priced accordingly), the frequency response is similar to the 650 but actually slightly darker up top, although this oddly offset by a driver that's known to present a pretty extraordinary amount of detail given the tuning. besides a few random QC anecdotes I've seen online, the build is supposed to be slightly nicer. 150 ohms (easier to drive). In the US, typically priced around $500. short version: most expensive version, but many who own it love it; others are confused by the exceptionally dark tuning.
drop.com collaborations:
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6xx: this has been confirmed via product teardowns to be a 650 with slightly different aesthetics. comes in matte navy colorway but otherwise sounds the same. 300 ohms. In the US, priced at
$220 and is an incredible value proposition. Typically goes on sale for black friday / cyber monday and drops a bit lower (I got mine for $165 new a few years ago). short version: no reason to get the 650 when this exists for much cheaper. generally makes it hard to justify spending more for the 600 or 660. my general recommendation for anyone with an amp or an interest in getting one.
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58x: unique creation at a lower price point and easier to drive. slightly more sub-bass than the others and less emphasis on the upper bass than the 650, slightly more treble than the 650/6xx/660, slightly less upper mid shout than the others. it is slightly less refined than the others overall: the treble has a slight grain or harshness that none of the other models have. 150 ohms (easier to drive). In the US, sells for $170 (also goes on sale; I got mine for $135 a while back). short version: insane value proposition, excellent starter audiophile cans for someone who doesn't have an amp or plan to get one. can be significantly improved with judicious EQ (people have compared against the 660 when both are EQ'd to the same target and not been able to tell the difference).