From a technical point of view, there is no reason for this to be true - the impedance is very high which makes them easy to drive, not difficult. The fact that you have to turn up the volume dial further than with other headphones does not mean they are hard to drive, it means they require more voltage (and in this case less current). In general, it's easier for amps to deliver more voltage than more current, and the power that all the HR amps (other than the mobile line) can deliver into 300-600ohms is an order of magnitude higher than is required by the headphones.
You should also consider equal loudness contours...
As you can see, our relative sensitivity to different frequencies changes depending on the volume. As you turn up the volume, with any headphone, the perceived balance tilts towards the top end, i.e. becomes less smooth and more strident. As you turn the volume up, if they are being driven accurately, even 650's should sound decreasingly smooth.
