I have a few amps. I'll bet you the FiiO E12 portable which can put out 880 mW at 32 Ohms with an output impedance of less than 0.5 Ohms can do a fine job. It's distortion figures are very low. I also have an amp that I designed that can put out enough power to drive an HE-6. I have no interest in coloring the sound by using amps that generate even harmonic distortion to "add warmth or one that rolls off treble and so on. IMO there's a lot of fun myth about amping and many have a good time fooling around, hence the hobby aspect. The FR of the HE-500's look superb and shouldn't require being messed with.
IMO, the main factors are THD, IM, TIM, transient response, slew rate, drive power and damping/impedance. Most well designed SS amps produce distortions well below the human ability to perceive. Yes there are some amps that cannot deliver but there are far more that can do a superb job. IMO too many people are getting overexcited and letting that get to their ears. Mag Planars tend to be resistive which eliminates the problem of varying impedance found on many dynamic cans. The HE-500 should drive rather easily, just as Hifiman states. I can handle the 89 dB efficiency rating without getting nervous.
If you have an amp that can drive the HE-6, then you should be fine. Tube amp distortion and warmth is a subjective thing, no doubt. Lack of power is not. I'm sure you're well aware of clipping. As for the frequency response being superb, I agree, though let me redirect your attention here: http://www.head-fi.org/t/646812/hifiman-he500-he400-jergpad-mod-v2-5-modulor-orders-updated-december
Mine should be coming in shortly.