| If the speaker sounds fine with a different amplifier, then the amplifier is most likely the problem.
First, make sure all connections in the leads from the amp to the speaker are ok, check the input plug on the speaker is not loose, check the internal wiring of the speaker is ok.
If the problem is with the amp, and you do not know enough electronics to repair an amp, then you will need to take it to have it repaired. Doesnt need to be an amp tech, someone who services hi fi gear will be able to do the job.
If you have a multimeter, check the output volts from the power amplifier. It shoudl be within +/-0.5V of zero volts.
If the power amp output is (say) more than 10V DC, or less than -10V DC, then an output transistor is probably blown, and these will need to be replaced, along with their driver transistors.
Output transistors need to replaced as sets, ie for the 370, if (say) Q307 is blown, then you woudl replace Q307, Q308, & Q309, along with driver transistors Q305 & Q306 | 
