While most airlines have eliminated the disposable hollow tube headphones, why some airlines use single-prong headphones while some use double? IMO it would make sense for IFE systems to all use single-prong headphones, as those are nearly universal on electronic devices.
On Delta, the Song aircraft use single-prong headphones, while the rest of the fleet uses double. With the new IFE system arriving, I would guess the fleet will switch to single-prong headphones, with the exception of the MD-90s.
Another question: Will Delta's widebodies use the Panasonic eX2 system? The eFX is used on the 757s, and is designed for narrowbodies.