When does speaker distortion become audible?My answer is simple. It's always audible.
Since it seems safe to assume that no speaker is audibly perfect, all speakers can be presumed to distort the signal that is provided to them to an audible degree. And most likely in multiple ways.
And, hypothetically speaking, even if there were a speaker that is audibly perfect in some rooms, it would undoubtedly not be perfect in many or most others.
Regards,
-- Al