Some headphones which I've sold have worked just fine here but had channel imbalance when they arrived.  Conditions they see in shipping sure plays a part here.  Just run them in some more and the bias supply should overcome the parasitic charge.   
 
  
 
I've used them at Pro bias on quite a few sets with no issues.  Stax also used the same cable (same internal wires too) on the SR-X Pro so it should be just fine.   
 
The way I did it was to cut the strain relief free from the plug and pull it higher up on the cable.  Cut a small opening in the cable sheath and find the middle bias wire and combine it with the 5 pin bias wire.  Slide the strain relief back into place, secure with glue and drill out the center pin.  Very fiddly job though...