David, the Dome Filter, or "prefilter" as you refer to it as, was slightly changed when the EXL came out. It is very similar though to the CXL/DXL/EX20 Dome Filter.
Erik, if you have the EXL or MG1 that uses the 2 stage motor, the Dome HEPA filter should fit fine. However, if you have the MG2, I doubt it will fit....starting with the MG2, they changed to the Aerus VM3 motor which is a single stage motor, & is junk! That motor is not well-made & has a VERY short lifespan, especially compared to the Ametek motors used years ago. They probably changed the size of the Dome Filter to accomodate the new motor, so the Dome HEPA filter for the old models likely won't fit.
Rob