marantz speakers problem

I have Marantz Challenger-7 set of speakers.

One works just fine the other sounds flat.I can tell that the large speaker works but the other two small speakers have no sound what so ever.


Where should I start to investigate the issues?


I opened the case and checked all wiring but it seems fine.


There is also some "plastic" looking cylinders inside, the knob that controls some sound color or something. and light that comes on when the speaker is over loaded and I think I sow a capacitor or something .


Any suggestions appreciated.