I believe the front speaker enclosure has 2 speakers including a tweeter. I will be willing to bet both speakers are OK but I would check for resistance anyway. But I think your problem is going to be the amplifier board. I have fixed several boards for that same symptom.
The rear speaker enclosure is one coaxial speaker.
Most of the capacitors that get replaced are de-coupling capacitors and are 1 or 10 mfd electrolytes. They can cause squealing and whistling if they open and are on the 12 volt supply points mainly. But there is one capacitor that is a 10 mfd coupling capacitor which when gets old the value of it goes way down and causes low volume. It is C172 (@16 vdc) and I think will fix you up.
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