Is it possible the amp could sound good and still have a cold solder joint somewhere along a signal ground path?Yes, that certainly is conceivable, Jim, although of course it would be very difficult to troubleshoot.
Marty, I realize that this is all probably moot, because you are proceeding to look for a replacement amplifier, but I see in the thread that you started today that you actually did mention nearby radio transmitters. It would be interesting to know how far away they are, whether they are am or fm, how many watts they are (perhaps that is indicated on their website), and if when you hear them through your system you hear actual music and/or voices, or just static-like interference.
Regards,
-- Al