Regarding the MDR-EX750/IER-NW750N, IER-H500A /IER-NW500N, IER-NW510N distinction, they are all Walkman A series specific earphones. Some could do binaural recording with Xperia. NW510N is the 6 mm driver IEM for NW-A100 series, with a strange shape that hides in the ear. MDR-EX750 is notable for its balanced circuit connected to a single ended plug. It is not clear whether later iterations had the same balanced circuit, or whether NW750N, its walkman version, had this. Or whether the sound improves due to it. But it is remarkable that my experience with using IER-H500A with A55 matched another user’s with their IER-NW500N. Perhaps due to A55’s incompetence, they’d stopped listening to it via other headphones, until they dropped into a Sony store and tried NW500N. Somehow the 9 mm driver IEM works well with A55, from a non-critical layperson’s perspective.
When would a person be able to tell that the combination is good? Perhaps it is no different to any other combination so long as it is also a 9 mm driver with similar specs. Perhaps it is the fit with the ear that differed. But overall, the effect is to allow A55 to playback sources without causing any dissatisfaction, and to have the user begin to feel that the A55 is really a music toy. Perhaps not outrightly fun or warm sounding but definitely something to play with rather than a professional sound.
