Planars don't care about output impedance.
Planars don't exhibit impedance variation with frequency is true because they are pure resistive loads. Dynamic drivers are highly reactive loads. This trait is responsible for perceived variation in frequency response and why dynamic drivers can have myriad of frequency responses based on amplifier's OI.
However, electrical damping is another thing and also for maximum power transfer to the load the OI of source should ideally be 0. I'm not complaining that Ananda had any unevenness in FR part from iHA6. I'm saying it sounded slow and bloated.