Glad you had some sucess and part of the drone is reduced.

Exhaust tips that are small mufflers will help a bit more.

I do not understand how you decided on the 25 inch length for the stub without measuring the frequency in Hertz of the drone sound beforehand.

Changing your fuel chemistry will probably change the frequency of the drone a bit
and/or the rpm where it is maximum loudness.
You could try “pure gas” without ethanol,
or create a test blend by adding some E85 to regular E10 gasohol.

Hotter summer air temperatures will also change the drone a bit.