Some people will call it an innovation, while the others say it is a crazy idea. A ⌀13 mm driver for the woofer, a ⌀8 mm driver for the mid-range unit, and a ⌀6 mm driver for the tweeter coupled into one IEM? That is 3DD, designed by Unique Melody, a well-known custom IEM manufacturer from the mainland China.
Oh, but they do not simply end there.. On the back of the woofer, there is an adjustable bass tuner made of a magnet, so that the damper, which is also magnetic, can stick on the woofer conveniently. As a result, the level of bass can be precisely tweakable, and the magnetic flux density of the woofer's magnet is effectively increased(thus its electroacoustic efficiency if the resulting B is any significant at all).
CON: The magnetic bass tuner is extremely hard to replace, especially when it is flipped upside down.
Manufacturer's website: http://www.custom-iem.com/
MSRP: $599 (-$100 for universal)