Well, I broke down and ordered.... a HE-560. But then, the HE-6SE deal came back, and I was real interested...
So I ordered one too!
First impressions: very good, but I felt there was some confusion in the upper range, something holding it back. I tried without the grills, and the treble opened up slightly, and that really gave it the air I craved after selling my Sundara, as well as helping instrument separation and placement. I wasn't going to keep it without the grills on, so I took the grills off my old (and dead) HE-5LE, on which I had removed the cloth (as was recommended back then to open up the treble) and I feel it is very close to the sound of them fully open.
So, quick HE-560 V4 vs HE-6SE V2. The HE-560 has a couple of areas where it is stronger than the 6SE: it has slightly stronger impacts, and it has a strongly more powerful bass. That said, it definitely lacks air and treble versus the 6SE; and while the impacts are stronger, they are much better defined on the 6SE. I feel the 560 brings drums forward in the mix, while pushing back vocals slightly. It is a very competent headphone, but it definitely lacks the very fast transient response, instrument separation, air and clarity of the 6SE. Curiously, I also find it less comfortable, despite the fact it weighs less. So, it is going back.
I'll probably post a HE-6SE V2 vs my other new acquisition for the end of 2020, the Focal Elex, eventually. It will be more in depth, of course.
For reference, I'm driving these with PC -> USB -> Soncoz LA-QXD1 -> XLR -> Topping A90 (med gain, just past 12 o'clock) -> XLR