My HE-500s measure at 51 ohms which I think is in the allowable range, I remember some people having phones measuring over 60 ohms. Mine was also one of the earlier production ones just after the Canare cable got phase out so don't know if the newer ones are closer to the claimed 38 ohms and if production quality has stabilised?
The Essence STX can drive the HE-500, but I didn't find it a particularly satisfying combination, but it will suffice if you are saving up for an amplifier. The Essence STX is better suited to high impedance cans. The 60 ohm output impedance on the NAD will probably not do it for the HE-500.