Etymotic BA iems are a special case because they rely on resistors built into the cable to tune the sound. I think ER4S is 100 ohms. I think they might've shifted those into the IEM body itself on the SR/XR, but I'm not sure. If the resistance on them is off or mismatched between the two sides, it will affect the sound of ER4S/P. Probably the only justifiable use of a nonstandard cable connector on headphones in this case.