A very strange headphone. It's likely that the bass control is merely a treble cut control and that the treble control rolls off the bass. Odd that the drivers worked, then stopped. I assume you bypassed the cable and EQ circuitry for your tests.
Have we ever suggested that you get a heavy-duty contact cleaner similar to the DeoxIT that's sold in the US? It's just possible that a tiny drop or two applied to the center electrode will help. Have you tried gently twisting the center electrode using the solder tab?