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...