What was replaced? A driver, power amp, cabinet...?

If by chassis you mean the cabinet, then I would suggest to check the sealing. The woofers are of the closed box type and if there is any air leakage, the performance will be affected.

try to reduce the treble of your audio system to the maximum negative value, choose a music track with much bass and drums, sit beside the repaired speaker and listen to cheep / wind noises from the chassis during drums / bass sounds. When the volume level is high enough you can easily hear it when there's air flowing that doesn't belong there.

And please care about your ears ;-)

So I had my BL20s repaired under warranty. When they came back home, the right speaker wasn't sounding as good as the left. I carefully paid close attention to both speakers and after very careful listening at multiple volume levels and different music tracks, I found that the woofer on the right speaker wasn't firing. The repair person came back and confirmed it. All it was, was that the wire which connected the woofer to whatever else is in there was disconnected, and had to be reconnected.

It could be as simple as a loose or a disconnect in your speaker due to some sloppy work by the repair facility.

In theory there shouldn't be a difference,

A parameters of the drivers and settings according any previous calibration are stored in a small IC inside the chassis.

That IC has been/will be placed in the new chassis next to the microprocessor.

If there are doubts whether that has done properly, the repairman can with B&O's servicetool download these values and store them again in the new chassis (and that would be in the same IC (Eprom). Each driver has its serialnumber and its "value". Both are stored on B&O's server.

The whole cabinet was replaced and the 'chip' transferred over.

Could be a loose connection. Ok, i'll let the repairman do his magic. Hopefully he won't charge me extra!😕

The wiring of the woofers at the front and bottom could have been changed by mistake. One of them is marked. But if that happened, that would instantly noticed when calibrating.