The main difference between models of the iDX series and the DX series are the remote control and the microphone integrated in the cable. DX 120 iE and iDX 120 iE are identical in terms of sound. The same applies to DX 160 iE and iDX 160 iE.
The iDX models include two extension cables that are used for connecting to devices with different pin assignments (4-pole). The DX models include only one extension cable (3-pole), as the pin assignments for devices are always the same in this case.
The functions of the iDX models are optimised for use with Apple devices (iPod, iPad, iPhone, etc.). Indeed, you can start and stop media playback, accept calls, communicate using the microphone integrated in the cable remote control and then hang up again on most devices.
Although, it is only possible with Apple devices to regulate the volume with the cable remote.
iDX models additionally include an adapter cable that can be used to also connect the headset to computers and other devices which have two separate 3.5 mm jack inputs (one for the microphone and one for the headphones).