Someone mentioned QED and I remembered they do publish
measurements of all their cables. Then I found
this white paper that explains why a power cable can make a difference and that includes some measurements. It seems the equipment details are missing so they can't be reproduced.
In particular, on page 5 there is a measurement of the the output of some (unspecified) integrated amplifier with a generic and their cable. Let me quote that part:
They have some other measurements of the output of a "typical integrated amplifier".