This is the first one where I see little difference in tonal balance between passive and active.
The ANC can be turned off in the 5909 (acc to Sean O.) and will still have the same tonal balance.
AFAIK this is the only headphone that does this (not change tonal balance without ANC).
Of course it will be seal dependent wihout ANC for sure.
This is most likely an artefact of the way the ANC feedback curve was designed. My hypothesis is that the passive curve (which was designed to match Harman decently well from the start) was measured on a GRAS hammerhead-style fixture, and then the ANC feedback curve was designed and calibrated to dumbly match that on such fixture. Hence why on these fixtures you see a decent match between the modes, but you don't as soon as other fixtures are used (SoundStageSolo, HeadphoneTestLab), and you probably won't when real humans will use it.
If the feedback mechanism is off, the FR will vary depending on various factors - not just leakage (ex : pad compression). When it's on it clamps down on these variations and maintains the FR stable. Unless your on-head response in passive mode happens to magically match the response on an ear simulator - and given that these are a closed back from Harman it probably won't for most people - you won't get the same FR between ANC on and off. It's just not even possible.
I can't do much better than this on that subject : https://audiosciencereview.com/foru...se-use-of-the-harman-curve.29633/post-1087051
The headphones that are truly attempting to deliver the same FR between ANC on and off to a large range of people are the AirPods, but that's just because Apple has found a way to have the feedback mechanism still operates when ANC is turned off (and the way it works means that it can be tricky to measure them with sweeps without taking some precautions).
Certainly not most ANC headphones do this digitally.
Is that referring to the ANC itself or to the match between the modes ?
A lot of Qualcomm's SOCs for example mention that the ANC is implemented digitally, but maybe it's a misunderstanding on my end. Ex : https://www.qualcomm.com/products/application/audio/qcc5100-series/qcc5125
"Fully programmable digital ANC"