With two pairs of speakers and your receiver all with a non-working channel, I am concerned there may be other problems. I suggest some trouble shooting before trying to replace parts.
First, disconnect all speakers. Then take one that worked (possibly the right channel from the Martin pair) and try only it on one speaker output at a time. The goal is to confirm that the receiver is working correctly and not the source of your trouble.
If the receiver works correctly on all speaker outputs, then try each speaker, one at a time on any output. Obviously, any that don't work or sound wrong need attention.
If any of the receiver channels don't work, check for fuses and replace any that are blown. Do the speakers have fuses? If so check them as well.
If none of this works, follow Thrifty's approach.
Good luck