Door amps

Evening All,
Is it possible for just the bass signal in the door amps to be knackered?
I cant get any output from the drivers side front and rear and passenger side front bass speakers but I can from the passenger side rear bass speaker.
Its the Harman Kardon system, can anybody help me out with any suggestions?
I note that there are always loads of door amps for sale in ebay, is it common partice for them to fail and if they do which bit fails?

thanks,


Dave