Just wanted to send a note over here, as I didn't see it mentioned, but HiFiMan has revised the HE400i design (more than just the 2.5mm cup jacks). Now there is no backplate behind the magnets. HiFiMan said that this was done to increase soundstage, clarity and reduce weight. They believed that after extensive testing a backplate wasn't needed for the HE400i's magnet array. The driver was otherwise unmodified. This idea was first explored with the HE400s because it had the weakest magnets of any planar. After testing HiFiMan felt confident adapting this design for the HE400i's stronger magnet array, believing that the casing was stable enough to keep the magnets stable without the usage of a backplate. It seems the design change was made simultaneous with the introduction of the 2.5mm jack, so if you have 2.5mm cup jacks you have the new backplate-less design as well. Most users seem to believe that, at the very least, it has improved soundstage and "openness", putting its soundstage on par with a HD600, and substantially better than the "old" design. Not much (if any) A/B testing has been done at this point, as understandably almost nobody owns multiple copies of the HE400i.