I can clarify it was my 6005 that was used.
I appreciate everyone's help so far, glad it's not just me pulling my hair out!
I am 100% certain I have the basic primary E and not the phono. I've checked every inch of the turntable and it does not have the line/phono out switch.
That concludes the enquiries.
From all that you've said, your Marantz amp has an intermittent fault.
Direct from your cartridge, It's phono input has been variously OK and unbearably loud.
Via a working phono preamp, its line input has been the same, OK and unbearable.
As you've proved, intermittent faults are the worst. They don't do service engineers any favours either - problems can be tricky to find and, sometimes, to reliably cure.
On the other hand, the cause could be very obvious on internal inspection.
If you decide to get it looked at, I advise you to try for somewhere that will do free estimates.
Importantly, check that there would be a guarantee on any repair that's done.