That last post was a mouthful!
So your problem setup is on an ipad. Presumably running JRemote? The DSP settings do not affect JRemote's local output to the device it is running on (the ipad).
I would test the same ipad, same headphones, and use a different source. The symptom you are describing (no vocals) is very similar to what happens when one of the wires in your headphones breaks.
If you get the same problem, on the same ipad, with the same headphones, using a different music source (like Amazon music, or ITunes, etc) then it's the headphones, the cable, or the ipad.
If it works fine with ITunes, but does not with JRemote, then something else is going on.
Brian.