Sadihyour probably going to want a more powerful amp , i have both an audioengine d1 and a fiio e10k and their what i would consider borderline able to power these cans . although take this with a grain of salt as i like really loud music.
SadihJust got my SMSL yesterday and used it with my HE-4XX, the sound is great put the amp is not really strong enough to drive it to very loud levels. I had it crank to max and the sound level was still bearable. Get a more powerful amp if you have the HE-4XX us my recommendation.