Sony XBA-3 is one of Sony's very first IEM models equipped with their own proprietary balanced armature drivers. Since the development is vital for Sony's future IEM production, 投野耕治-san, a deputy general manager of Sony Electronics in Japan, who is also the master of Sony's MDR-series, takes charge of the entire development process.
And eventually, 投野-san comes up with total of four XBA models: 1, 2, 3, and 4.
"The ability to combine woofer and tweeter drivers in addition to the standard full-range driver, would also greatly appeal to music lovers seeking an even more intensely satisfying listening experience, with bass and treble frequency ranges reproduced in even higher-fidelity."
CON: Impedance characteristic is not linear at all, which indicates a need for low acoustic output impedance source for consistent damping. Also, the peak at 4 kHz is residual, and is also quite audible.