Really wasn't a fan of the HE4XX, but earlier I discovered that Vincent's KHV-111 tube hybrid made them much better, like it tends to do with all headphones. Smoother treble, more body and low end.
On the Gustard H20 they absolutely come alive though. They really seem to like power and respond well to a high quality amp. The H20 transformed them - much better resolution, smoother treble and much improved low end, better imaging & wider soundstage and a slight warmth to the mids, akin to honey to the ears. All in all a fuller, more accurate, more musical sound. I'm not sure if I should be impressed with the headphones or the amp here. Both I guess? I had no idea the HE4XX had such potential. It just took an extraordinarily exceptional amp to reach that potential.
The HEXv2 will remain my daily driver, but knowing this I'll definitely give the HE4XX a spin every few days, until the H20 is sold. After that I don't know if I should keep them - it might suck, knowing how good they can be with the H20 and not having it.
edit: I was running the HE4XX balanced with a custom silver cable, if that even matters. I'm not a believer in cables changing the sound, however at least on the H20 the balanced hp out is clearly superior to the SE.