The larger muffler added on in line with the tube Briggs stock muffler, is it store bought or your own design?
I know that automobiles had in the past maybe still now or not, I have not really checked recently, a separate resonator that is after the catilic converter and before the muffler to change the frequency of the exhaust to a lower tone.
This is maybe what you have achieved. The length of the whole exhaust system also has an effect on the sound, so you have a good time not pissing people off if you’re a baker going to work before sunrise.
I used to do that for a while leaving at 4am and pushed a road bike motorcycle that had a leaking manifold a block to get into a place where it was furthest from residences before starting it. I tried to get a place to weld it but no place was interested so it goes.