It was wild ride with HE-400, really liked them when I first heard them. But after some time I got Schiit Magni and Modi and when I heard HD650 of this combo I absolutely loved it, spent lot of time with them and than when I put back HE-400 it felt so wrong to me that I never got used to them again and it was kind of over of my story.
Bass is definitely amazing on HE-400 no question about that, but mids just don't do it for me compared to HD650, I think it's because HE-400 are more laid back, I found out my preference is leaning towards forward headphones and I can't turn them too loud because treble on them would destroy my ears.
There is just something magical about well amped HD650. Can't get enough of them with Schiit Lyr.
Yeah the HD650 is like a sleeping giant when underamped, glad you're enjoying them.