Information about jumpers:

Jumper A and B are used to control audio effect of bass range. A for lower limit while B for upper limit of the bass audio range. For some speakers, when Bass is turned up too much, the speaker will show distortion. When that happens: we remove the jumper cap of jumper A to limit super low Bass to bring distortion lower. When we remove the cap of jumper B, Bass is increased while cap of Jumper B is on, Bass is decreased.
Jumper C and D are used to control Treble range. When cap of both jumper C and D are on, the sound of treble is loudest. When only cap C is on, the treble sound is lower. When only cap D is on, further lower. When both caps of C and D are removed, there is no treble sound. (Generally speaking, with the four jumpers, you'd sooner or later find your best sound effect. Anywya, all effects still depend on your actual test.)