Case of the amp is not earth if only wires you connected to it is the power supply which is does not have earthed output but floating.

So your setup 2 cannot work as it does not power the amp.

And it will be earthed by scope ground as probes are connected to the PSU black output.

Case of the amp is not earth if only wires you connected to it is the power supply which is does not have earthed output but floating.

So your setup 2 cannot work as it does not power the amp.

And the amp negative will be earthed by scope ground as probes are connected to the PSU black output.

The case of amp may or may not be connected to supply input, but you can figure it out with a multimeter.

Case of the amp is not earth if only wires you connected to it is the power supply which is does not have earthed output but floating.

So your setup 2 cannot work as it does not power the amp.

Case of the amp is not earth if only wires you connected to it is the power supply which is does not have earthed output but floating.

So your setup 2 cannot work as it does not power the amp.

And it will be earthed by scope ground as probes are connected to the PSU black output.