But the resistor value is 8ohms, which is pretty standard. I'm not convinced that headphones will come across the bit with the 100ohm resistor. The value should affect the sound somewhat, though I'm sure you wouldn't know unless you were looking for it specifically. Sort of like the "low" and "high" impedance settings on those chunky cabled electret headphones of old.
If I remember correctly, the standard used to be around 100 ohms for older amps, instead of the 8 ohms of today? Could be wrong about that. I imagine certain pairs wouldn't sound exactly right on a modern system. Dt48 comes to mind.
Those planars look like they have very thick traces, I wonder if that's bad manufacturing?