I have crappy power with THD often at 8 or 10%. Using a PS Audio PP12 regenerator made a huge difference, and then I moved on to Accuphase PS-530 and PS-1250 power supplies that eliminate THD and produce a perfect sine wave from the distorted wave coming in from the grid.
The impact on my audio system sonically is not subtle - it makes a significant difference in blacker backgrounds, resolution, dynamics, musicality and bloom. I have a solo piano recording (Schubert sonata) made in an old stone church in rural England, and in the quieter passages you can hear the birds chirping outside. I only heard those birds after installing the power supplies in my system.
The Accuphase power supplies are massive and built to much higher standard than the PS Audio units, and they are silent and free of transformer hum. They are also at different price point, but worth every penny.