Regards .... mm ok the problem that the volume level decreases even if you raise it to the maximum it seems to me cause of a damaged filter check the section of amplification and check the filters ... the problem of the bass speaker seems to me that it is the same speaker that is damaged or has a false contact in the internal conductor cables remembers that the bass speaker moves much at high volume and that movement over time can cause false contact in its positive negative pole or even the power with which may damage the internal coil of the speaker causing distortion in the sound. Open the equipment and disconnect the bass driver and test if the volume stabilizes on the other speakers and try another bass driver instead. If you do these tests you will discover where the fault is located and you will find the solution. Good luck and tell us how it was