If you connect a 'line out' to a PA input, it should work just fine.

I hope you're not connecting the External Speaker jack to the PA amp input.

The guitar amp speaker output needs to have the right kind of load connected to it, or things go badly wrong. Tube amps are especially sensitive to load mismatch.

At the other end of the cable, the PA input is probably designed to handle less than a volt. You may be feeding it 20 times that much!