I am very pleased with a sample build I have of an MLTL
(Mass Loaded Transmission Line) as given by the designer
who goes by the handle of GM.

I get some cone breakup when listening to an over the air
FM signal. If I change from stereo to mono the noise goes
away. What is the answer to this?

The details are: FE127e in the MLTL w/no stuffing or internal
damping at all.