Unplugging a cable with phantom power on has lost me a few pre amps over time. Possibly this may have happened to you. 
This is why it's nice to have a set up where you do not have to swap cables. You learn the hard way. Even with phantom power off, when ever I unplug or plug in something to the 6i6, combo jacks, the peak meter goes red. I'm in the habbit of turning the level right off now to prevent this. 
The 2i4 might still be usefull for line input. Save it as a spare.