Thanks @oneofmany, I find it interesting that we had similar experiences on very different devices, despite both of us expecting something quite different. I wish I understood enough about all this to know what the most likely interpretation is. As for the iPower itself, I know that elsewhere on the forum both Allo and John Swenson have expressed misgivings about its performance - I’ll be interested to hear how Allo’s new PSU (Nirvana) performs when development has finished. FWIW I now use that iPower on an IQAudio Dac+ to clear benefit over the stock RPi PSU.
On a whim, I tried powering by battery (tp-link tl-pb20100) to eliminate mains noise - as I understand it these are still SMPS though. No brightness but neither was there any improvement in soundstaging, if anything it was worse and the music sounded less impactful. In addition, it seemed to generate audible HF noise from the DAC itself so definitely not a keeper.