Excuse for my english im dutch.
The Fiio E7 might help the Cowon Z2 a small bit. The Z2 does not have Line Out, so you will have to go out of the headphone jack to the E7. I remember the E7 as being not very strong -- I had to turn it all the way up to hear an increase in volume. The Cowon players ar usually well-powered, but don't know if your Pioneer headphones need lots of power.
The DAC portion of the E7 did work well out of my computer, if you need that.