Well, I have suspected that certain global parameters such as global equalizers were changed by a previous user or in a music store, making the unit sound weird and bad.
Now you have reset those.
Aah, thank you Christophe; this resetting of parameters that had been adjusted without Duncan's being aware of it was the first thought I had, but I suspected data corruption because the symptoms didn't sound like anything I could explain this way.
It's a pity we're not able to hear samples of the "before and after" situation.