Null test with music.
I would null test with music after removing Raytheon RC5534 op amps from SSLs and replacing them with Signetics NE5534s.
A pair of RC5532-equipped channels wouldn't null well with each other and the residual was pretty gritty.
A pair of Signetics 5532 channels would null well and the residual sounded like the attenuated source.
It was pretty convincing for the client to hear the difference.
I couldn't have done this running tone.
I replaced thousands of RC5532s at no charge to the client.