Some faults don't have a single cause - they're caused by an interaction between 2 (or more, $DEITY help us) components. Thus swapping/altering either component can this fix the fault, and it would be easy (but wrong) to claim that this proves the changed component was the faulty one.Hi Graham, I don't think it's the amp at all, with the pi disconnected and using an old cdp as source it's completely silent. Sadly I don't listen to cds I could maybe try my active pre (Exposure 7) in place of the passive but that adds its own flavour over the passive. With the Qudos I was running the pi directly into it, no pre at all.
A trivial example/metaphor is trying to fix a 6 hole wheel onto a 5 bolt hub. It won't work, and you could make it work by changing either (or even both...) components.