In general, when transmitting audio between devices, such as from a console to a receiver or a TV, you want line-level audio, because that’s the specification in place. Amplified audio, on the other hand, is intended for directly driving speakers, and thus has way more energy than line-level audio.
So, the effect of running amplified audio into a device that calls for line-level audio can result in permanent damage. That’s precisely what was happening with the gscart switches (lite and earlier models that don’t have overvoltage protection) when used with the Windy [City?] Gaming supergun with the unlabeled switch set to output amplified audio. It’s possible your stereo amp has some form of overvoltage protection, which would be why it continued to work after feeding it amplified audio.
Still, a supergun should never be putting amplified audio on a SCART output–it’s simply way out of spec for SCART–and users should not need a hi-lo converter for an output that should ostensibly conform to the existing standard. If someone wants amplified audio on their SCART cable, give them some screw terminals and let them wire it up themselves.