The XO will be optimised for the HF diaphragm and LF cone originally installed. Some Cs have an early cone, some the later E style with pleated surround. I think the part number for the HF diaphragm is the same in the C and E, so strictly speaking, you could interchange the XOs if each driver has the similar LF cone in it.
Or you could get a pair of the GPA XOs. Or wait till a mate for yours turns up on eBay, which happens.