This may be conformatin bias on my part, given the following train of thought. Lower frequencies have much longer wavelengths -- that's why the bass pipes on a ppe organ are 16' tall, and in some cases, 32' tall!! With conventional speaker drivers, the driver's dispersion gets narrower with increasing frequency within its usable range. You need larger rooms to support lower frequency reproduction, as the waves are bigger....I was thinking that the smaller bore size of a tip attenuates the highs only, and by lowering the highs it would create an illusion of enhancing the bass.
So, it seems to me that bass needs a bigger aperture to come out. I would be quite curious to know what iem desginers would say in this regard - does the iem tech behave similarly to speakers in rooms?