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That is the first FM radio that I have heard of with this type of detector. I have quite a few tube type FM radios and they all have either the Armstrong type detector or the ratio detector. I have also seen the schematics for many more and never seen this. I guess that their are always exceptions to the rule. There were some TRF table radios made until around 1950, long after the superhets were mainstream-again to save money.
Zenith used the "Armstrong System" for their FM radios and paid royalties. Other manuafacturers used both.
Cost was the main reason, the quadruature detector saved 1 tube amplification stage. I think that Zenith first came up with it, but am not sure.
It would be interesting to compare the FM radio with a Quadruature detector against one with the Armstrong System while a thunderstorm was nearby. This would be a good test of the AM rejection.
Another reason that the sound output of a TV detector would not be as good as a CD is that TV sound bandwidth was limited to 10KHZ, not 20KHZ as a good stereo is.
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