More discussion of this thread at CA. PaulM suggested continuing discussion here, and to facilitate that, I've copied some of the key subthreads on this page. There are related threads here and here. Tallbloke also has a thread here
In one of his recent comments Bart posted this diagram of the system he was modelling:
It's a standard feedback system - just two boxes. But to me it raises this query - how can you isolate the box that you want? Electrically, if you look at input-output of box T2, you can't avoid the fact that T1 is in parallel with it. So you's disconnect the box and do the impulse response or whatever out of circuit. It seems to me that weve been trying to do that analysis in circuit, and getting tripped up with causality issues.