I’ve just bought a pair from the Sennheiser shop on Amazon U.K. Given the comments above £139 seems very good value for a set that need such minimal eq to achieve Harman, and will be mostly fine without.
It really is stunning value at that price! They're a difficult headphone to fault, and most certainly for that deal. I think you should find the bass most pleasing & surprising after EQ for an open backed headphone, and the rest of the frequency response should be fine too.
EDIT: if using something like an Oratory EQ, and if tonality is not spot on have a go changing the tonality using a linear tone control slope across the whole frequency range, three High Shelf Filters with Q0.5 at: 63Hz / 632Hz / 6324Hz. To decrease brightness you'd use -(minus) dB on each of those, so always the same dB value on each one, you'd start by putting increments of only 0.2dB changes to each of those, so you'd do something like -0.2dB / -0.2dB / -0.2dB for each one if you wanted to decrease brightness and increase bass, then if it's still too bright you then might try -0.4dB on each of those filters (so 0.2dB increments each time - because the filters cover the whole frequency response then you will notice the effect of just 0.2dB changes on each of those three filters). I'm only really mentioning this tweak in specific relation to this headphone. I think it responds well to a linear tone control EQ, more so than my other headphones, MayaTLab (a user here) did measure his in ear response of the HD560s headphones and they can show a positive slope in the mids that you wouldn't expect from the GRAS measurements, so my loose theory is that a linear tone control can bring back the slope of the mids to something more ideal. (You'd use this linear tone control on top of the Oratory EQ if the Oratory EQ tonality wasn't spot on.). I wouldn't normally suggest this EQ approach for other headphones, just I've found it works well for this particular headphone. Something to think about, try it if you like.
EDIT#2: praps think about
not using Oratory's High Shelf at 10000Hz if you take my approach of using the Linear Tone Control I described in the previous paragraph. So delete Oratory's 10000Hz High Shelf if you try my Linear Tone Control approach.