Random thought: Rephrasing

You have a device running on 115V.

When you use it with a step-down transformer, you worry about the negative impact of the step-down transformer.

But when you use it with an isolation transformer that also does 230V -> 115V, you start hearing the improvements.

Note that some step-down transformers are autotransformers which do not offer isolation. However the step-down transformer inside your consumer equipment is almost always isolated.