but again all a stereo amp does is increase the sound from the head unit - it's got nothing to do with receiption of an FM signal

the antenna (or arial) amp helps increase signal strenth

best of luck with your fix BUT something doesn't seem right that by swapping stereo amps your FM signal comes back

 

Don't forget about the wire that switches on the power to the amplifier when the head unit is switched on. Somehow some sort of RF is being transmitted back from the amp down this wire which is affecting the FM reception. The installer had never seen anything like it in 20 years, but it is definitely something to do with the design of the amp.