If you have a falling mid range and highs , your panels are on their way out. You can try and dust or vacuum them real well and see if it helps. If they are original panels chances are they are done. If its one speaker then swap the panels from one to the other just to make sure the crossover or power transformers are good. If the low sounding panel fallows with the switch its a panel if it stays on the original speaker with the switch consider the crossover !
To disassemble , take a mallet and hit the wood rails up wards from the bottom. They are on a tapered pin system that is simple to figure out once you see it. There are 3 wires to disconnect at the back of the panel . Disconnect them and switch panels. Do not be afraid to hit the rails hard they are on good. Make sure the speakers have discharged for at least 12 hrs.