It's not 100% a stability thing. The issue is worse with solid state amps, but the rise of output impedance does change the FR (which has been measured and proven in the past.) A relatively clean headphone won't be ducked with much, but a driver that cant handle the boost will get more distortion. My HD580, for example, got brighter and bassier out of the 600ohm. The Auteur got bloated a bit and darker. It just depends entirely on the headphone.