This forum is a goldmine of good information. I was at a race meeting today and an XS sidecar (number 611 in my avatar) was having problems with his midrange,
He had changed the muffler recently and is running a home brew 2 into 1.
The muffler looked similar to the one on your bike but black. I suggested removing the baffle as a quick test. Next race all smiles again.
I will suggest he goes back to the old one which was only changed for ascetic reasons.