Sennheiser frequently discounts their products. And some vendors recently had the HD550 for $200, and the HD560S went as low as $140. That is the lowest I’ve seen them go new in the box. Most vendors are now charging the MSRP, which is around $350 and $280 respectively. (There is one still offering the HD560S for $145 though.)
Both headphones are worth a try at the discounted prices imho. I have a harder time recommending them at the MSRP though. Because both headphones have some strong and weak points imo.
I have not personally compared them with headphones like the HD600, HD6XX, and HD650. Some of the reviewers here have though, and their impressions of the HD550 have generally been pretty favorable…
I think they should give the post-2022 HD560S a listen and some new measurements as well. Because I think most of their impressions of this headphone are probably still based on the older version with the 3 meter cable. I doubt they’d knock the newer HD560S with 1.8 meter cable down as easily as the previous version. But could be wrong about that.
I have a few gripes with both the HD550 and HD560S. I don’t really like the fit, feel, and pleather headband (sans divot) on the HD550 as much as the HD560S, which is quite comfortable. But the HD560S seems to trap more heat around my ears and head. I also don’t like the single-sided cable on both headphones.
Sound-wise, I think they’re both decent. But I also have some minor gripes there as well. Main thing I don’t like about both headphones is the lack of sub-bass. They have decent extension in the low frequencies for lower-cost open-backs though. And the sub-bass can be increased a bit with EQ on both.
The HD560S is more forward in the upper mids. And gets a little strident there. Some listeners might like that though. Maybe some older listeners who’ve lost some of their midrange hearing, for example.
The HD550 has better extension in the treble. However, I think the treble is probably a little overdone in some spots. So it can get a bit sibilant at times, and overshadows the mids and bass a bit too much.
Both headphones have some sibilance btw, but in different spots. It is probably correctable though with some careful EQ.
I can’t really say which headphone is closer in sound to a neutral loudspeaker. Both are likely in the general ballpark though. Sound-wise, I think the difference boils down to whether you like more upper mids, or more treble. Because that is the real tradeoff on these two headphones imo.