While I can wholeheartedly confirm that my HEX sounds just brilliant out of either my Mojo or AK240SS, it does still scale with my Hugo TT or Hugo TT/GS-X Mk2 combination. It is a brilliant headphone and the ability to have incredibly good (and flagship worthy) sound with you around the house and nothing having to be sitting at my desk with my full setup is a huge bonus. But make no mistake about it, these cans do scale with better gear.
But I do find that the HE1000s do indeed scale higher, but I don't really use them in situations that I use the HE-X headphones (because I can't really as they really need the power from my GS-X Mk2 to really sound their best). If you're looking for a great pair of top notch headphones and don't want to spend a bundle on the front end rig (like you would with the HE1000s, HD800S, LCD-4s, etc...) the HE-X are a great option to consider IMO!