Fisher Intergrated Power amp CA-270.

Yesterday the left channel and speakers went out.
This occured after the speaker selector was changed to a position where no
speakers were attached.. Could the lack of resistence have caused this?
This is a 20 year old unit that sounded great.
There are 3 speakers in the cabinet,how do I check if any are still good.Do
I check for resistence at the speaker cabinet connectors?
Also what do I troubleshoot at the amp.
It does not pay to send it out for repair and I am trying to save it.


Thank You