[morzh]

Have I done as you suggested? Nope. I am at a loss to understand why the signal is clean to the output with a signal tracer, but not with speakers or headphones.

Exactly what the scope is for. Sometimes trying to model thing in one's head is not working. What if loading your control amp distorts the signal? The scope will tell you if it is the case, and you can trace it further down if needed.

Well, when you have a chance, keep us posted.