You might be right, I have only used the HE400 on a few DAC/AMPS so I do not have the experience to say, but in terms of power they definitely do not need a lot to sound good, and out of the DAC's I have tried the differences are minimal, Xonar STX does a decent job with the HE400 but you are probably right that a nicer amp will give some improvements to sound quality, but I would not expect any massive differences, like with some headphones where NEED a expensive / powerful amp to sound good. Probably the HE400 paired with a smooth sounding amp would make them sound better.
IMO, YMMV, etc.
In terms of power, the HE-400 doesn't need much. My laptop and iPad can both drive them louder than I want to listen to without distorting, and my Clip can drive them to a decent volume without distorting. Out of the headphones I've owned, (DT-990 250 ohm, Q701), I'd say the HE-400 needs the least power.
In terms of sound quality, the HE-400 benefits greatly from a nice amp (as well as DAC). Bass extension, soundstage width and depth, and image separation all improve greatly on my main rig vs headphone jacks of various devices. Perhaps not massive differences, but very clearly noticeable. Out of the headphones I've owned, the HE-400 shows differences in amp/DAC quality the most.