Thanks for the test, Don. I have dozens of Murata 10.7 MHz filters from 110 to 280 kHz bandwidth. Some show little difference when reversed, but most do. Incidentally, it is probably the difference in group delay that causes the difference in audio distortion I observe when reversing a filter. FM detectors are sensitive to group delay. Try more filters if you can. If you could post forward and reverse .s2p files for 50 ohm drive, that would help me check my program.
