The XBA-4 is a superb-sounding pair of headphones -- they offer a keenly balanced mix of deep bass, a natural-sounding midrange, and smooth treble detail. The ultimate test of any headphones is how they sound after long listening sessions, and the XBA-4s never left me fatigued. Beyond the sound, the earbuds are comfortable to use as well, though I was aware of the bulk of the relatively large earpieces hanging off my ears. That said, they felt secure, and never fell out without a significant tug. The cable was free of "microphonics," meaning it doesn't transmit any sound when it rubbed against my clothes. Finally, heavy distortion and loud volumes pose no challenge to the XBA-4s, and I found myself playing music at deafening volumes without fear of distortion.
I compared the XBA-4s to the Etymotic ER-4PT (single balanced armature) in-ears for contrast -- the XBA-4s presented a richer, fuller tonal balance with a more powerful dynamic slam with well-recorded drums, while the ER-4PTs gave off a brighter sound and superior resolution of fine details. On the other hand, I actually prefer the way the ER-4PTs reproduced the snare drums attack and the cymbals' sizzle. They're both good, but I felt the XBA-4s prevailed with a more accurate and refined sound.
I also compared the XBA-4 headphones with the Audeo PFE 132 in-ear (single balanced armature) headphones. The XBA-4s had better bass and came off more dynamic and noticeably fuller-sounding than the PFE-132s. Additionally, the acoustic guitars sounded more realistic using the PFE-132s, with the XBA-4s blurring the sound of each note together.
All of the above listening tests were conducted with my iPod Classic, but I also used the headphones to watch a few movies on my computer. Paired with a FiiO E17 headphone amplifier, the XBA-4s sound even more spacious than with music, and add a deeper audio layer to the theater experience.
Sony's first foray in audiophile-quality, balanced armature headphone design is largely successful, and the XBA-4s sound excellent -- if you can swallow the price tag, you'll enjoy their sexy design and rich sound.