I am willing to consider that the AKGs are like hooking some really really good speakers to a given amp, that might reveal some shortcomings that you might not notice on lesser headphones. Details like soundstage, imaging, etc. that some 400$ speakers might gloss over but some 10,000$ speakers might make more evident. Therefore I suppose a power amp can drive them in most cases, but you might be hearing the amp instead of the phones. Since nearly all amps use a deal like a couple resistors per channel to attenuate the headphone output level then you could maybe change the resistor values if it seems to be too low at medium volume control settings. I would think most stereo amps create the least distortion somewhere above one watt output but well below full power output, so you could juggle resistor values in the headphone network to get your full volume in a sort of sweet spot in the amps range with a given set of phones and the variations of sensetivity and impedance that could be. They design the jacks(on amps with jacks) to work with anything that someone might have, between 8 ohms or 600 as a comprimise, I am sure. If you hooked phones directly to a power amp without the attenuation, you might hear hiss and hum and maybe crossover distortions in the background but if too attenuated, you could have the amp working closer to its max output and into the higher distortion specs.
