What happens when you plug two outputs together?

For people who don't have test equipment or know how to use it, your next best troubleshooting method is to do swapping of parts to find the guilty device. Have a known good signal line level source, a known good phono level source, a known good speaker, etc.

Divide and conquer...

Sometimes you might find a cheap line output source at a yard sale of something, or borrow one from a friend. It doesn't have to be high quality, it is just for test.

What happens when you plug two outputs together?

BANG!

=-}

No, seriously, it depends on the device. Some outputs have absolutely no output protection, some have protection for limited time (like 5 second overload, 30 seconds, etc.), and some have "infinite overload protection".

A device might be advertised as "infinite overload protection" and still be blown, however, because that protection was ONLY for accidentally shorting the output to ground, and not into another power source.

It's just not a good idea to do that... =-}