Andy Burns wrote:

If you copy the file over and over, it'll retain the same quality, but if
you keep-re-encoding it, e.g. to mix in other channels each time, you'll
get gradually worse...

Thanks for that. I'm using a multichannel mixer (MixPad) so I can keep all
the components separate and just export the mix down to a single file when
I'm finished. But since many of the separate tracks are mp3 files the
subsequent final mixdown does represent a first-generation dub of many of
them. But you confirm my suspicion that no deterioration is involved.