Something like this I think. Haven't tried it but it I think it should be according to schematic. Looks interesting though, I might give it a go. The amount of bass boost is controlled by that 15k. Recommended values to try, according to STM, is: 4,7k (0dB boost), 10k (3dB), 15k (6dB), 33k (9dB), open circuit (12dB). Or turn it into an external control.