Reviewers often comment that it takes about 100 hours before a new amp
settles down and start to sound like it 'should'.

In this case, what constitutes 'burning-in'? Is it sufficient to just
leaving the power on for a week? Or does it have to be driving a load
(the neighbours are going to just love that)?

Maybe a couple of chunky resistors across the outputs would be ok?

So use it for the first few weeks and hear it improve.
What are you, so precious that you can't listen to it until it's
reached optimum?

It's all bull**** anyway. As has been mentioned, there are other
electronic applications much more demanding than hi-fi. If burn-in of
components or cables, cable polarity, or any of the other snake-oil
existed, they would have discovered them.