When my preamp and our baby monitor receiver are plugged in to adjacent sockets on a 4 way the pre interferes with the monitor. Swap the monitor onto batteries, plug it in elsewhere, or switch the pre off and the interference drops away.

So it would be easy to see whether a 'fancy' mains lead made a difference to the amount of pollution reaching the mains from the preamp.

Which would be interesting in itself, but obviously wouldn't say anything about perceived sound quality.

Paul