Not disagreeing with you either regarding damping factor (maybe your post wasn't the correct one to quote).
People avoid high output impedance amps because of damping factor concerns, once they hear that planars aren't really affected by damping factor they then think its OK to get high output impedance amps. Just trying to point out that this isn't always the case.
High output impedance amps are not suited for low impedance headphones unless they can provide the required current. The exact same amp but with a low output impedance will push more current (for the same power output) and work more efficiently when matched with low impedance headphones. Its not just about power, its about current for planars.