Dan Mason

Thanks for the quick reply. It still shows 8520.

I’m happy to say this is now resolved.
I tried the LR-16 in a different USB socket and the firmware update was successful.
The crackling is gone.
I put the LR-16 back in the original USB socket and the crackling came back – so this was actually not a firmware or driver problem.
The original USB socket I was using was on an extension; I have used it before with other audio gear (e.g. USB mixer) without any trouble, but clearly it was not of high enough quality for this particular device.
I mention it here so that anyone else with the issue in future might see it. Might be worth adding USB extension cables to the FAQ also, as an additional possible explanation of crackling noises.
Many thanks again for the help.