I have a Hohner Morino Artiste VI D that also has a bit of a growl. The bass reed block has a U-shaped chamber. Would this be similar to the umlenkstimmstock you mentioned in your comment?This bass block uses an "Umlenkstimmstock" which makes for a much longer resonance chamber. That may contribute to the growl as there is a larger volume of air inside the reed block which offers less "counter-pressure" against the reed that still wants to vibrate despite the pallet being closed.