you have dried out electrolytic caps in the power amp.. particularly the ones used for DC offset stabilization in the diff amp of the amplifier board. these are usually about 100-470uf caps between the base of the inverting input and ground. if they are dried out enough to change value, you will lose bass response from the amp.
the diff amp is a pair of transistors with the emitters tied together. one side is the noninverting input, which is where the audio input goes to. the other side is the inverting input. this is where your feedback goes to. there is usually a rather large (compared to others around it) cap, usually between 100 and 470uf, and usually the voltage rating is 50V or more, that goes between the base of the inverting input transistor and ground. it will have a resistor in series with it, usually somewhere between 1-3k.
btw, did i mention i hate working with .rar compressed files? this is the 21st century, we don't need to fit stuff on floppies anymore....