Will that other Lunchbox sound different?

I recently got into a debate that triggered a call for analysis. Are lunchbox PSU’s/Shielding drastically different between products? Curiosity got the best of me and had to find out. I use three different manufactures myself API, BAE, & RND. Being all respected brands I wasn’t expecting to see drastic differences. Two of three have built-in PSU’s and thought that might have some impact.

For the test I used a Api 512c mic-pre set to amplify a – 40 dBu signal up to 13 dBu. Figured this would give a good opportunity for some noise/interference to sneak in. For a 2nd comparison I used the very clean Grace 502. I used it amplify a – 9 dBu signal up to 13 dBu.

The results showed little to no audible difference between these boxes. Below are the measurement plots of the experiment. The minute gain differences are do to manual knob adjustment.