You're hearing harmonic overtones, not the fundamental. The KEF M200's are actually a good example of this, because their bass distortion is quite low. You can't really hear anything when playing a 20 or 25Hz tone, which I consider impressively low distortion. A 40Hz tone can be "heard" through them.
Also, I never said they couldn't produce them. They all do, at fairly high SPL due to pressure vessel gain. The human auditory system can't perceive them, however.
There are lots of witchcraft-like beliefs on head-fi. For example, there is a large contingent of demented electricians who can't see a part without wanting to run new wires through it, who then ascribe magical powers to said wires. This one is slightly different. Human physiology is complex, and the obvious fact that one hears something when playing low tones through an IEM (again, harmonics not fundamental) can trick people who don't have a lot of experience enjoying systems with volume displacement capable of reproducing deep bass fundamental tones. (For the record, my primary home system has five underhung-motor Aurasound bass drivers, with a combined linear volume displacement exceeding 14L.)