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If you measure the difference between batteries and a fairly normal power supply in a phono amp which is very sensitive to these issues you will find very little difference, maybe a few db at something like -128db. It's really theoretical.
On the other hand if you have 2 items plugged into different power supplies and connecting them together by USB gives you a ground loop then it's more likely to be affecting normal levels of music. It's also common for cheap USB devices to pick up on rf noise from the computer.
I would recommend you measure different problem noises and decide what you can live with.
Airhead
OK, good to know. On your point about digital sources, and I'm not being in any way pugilistic about this, just want to know your take on it, isn't the difference between digital sources all about RF separation? E.g a streamer which has only 1 job to do vs a laptop which has a bunch of other processes generating noise. And if that's of benefit then something like a rPi steamer with a wall brick outputting directly from its own USB out vs a rPi with an isolated output board and low-noise power supply should give better SQ?