You can get rid of almost all FM hiss by dropping stereo. Does your receiver do that? Mono FM is remarkably hiss and interference free. I had a very old Dynaco receiver that would drop stereo with a switch. It's amazing what 20dB less noise will do for you. I did all my radio talk show recordings like that for years. The basic mono FM channel is 20Hz-15KHz and I think the proof noise floor is something like -55dB. Stereo was the problem and widely condemned when it was introduced.for removal of FM hiss, to use a notch filter?