Thanks, Dan. That didn't work either. You mean 56h (hex) or 0x56, right?
Yes.
When downloading from the radio, the channel numbers appear highlighted in red with an exclamation point ahead of each one, presumably denoting an error. Nothing appears in the rest of the spreadsheet.
I did notice that the error indications showed up a lot faster when the address is set to 0x56 than when set at 0x66, so at least there is some indication that changing the address had some effect.
The driver is marked as "746" not "746Pro" so it's very possible it won't work for your model (I didn't write the driver so I'm not positive). The presence of this bug indicates that's the case:
https://chirp.danplanet.com/issues/2157
If the memory format changed at all between the two (which would not be unusual) then it won't work and the behavior would match what you describe. Effectively "I got a response but it was not what I expected, so error" when the address is right and "I never got a response" when it's wrong.
You're welcome to add your debug log to the bug above, which will give some hints for someone who might come along and want to implement the 746Pro. See:
https://chirp.danplanet.com/projects/chirp/wiki/How_to_report_issues
--Dan