I have an idea. Humor me here, it won't take long and it just might fix your problem.
I too, have a Sonnet - different model, different unit (MDD) BUT
There seems to be a common issue that often causes trouble with these in OS9
My MDD refused to boot in OS9 until i was given this file to replace the one that was native to OS 9.2.2
It appears - on the surface at least - to be nothing more than a standard "Apple CPU Plugins" file although an earlier version than the one I had.
All I know is that when installed in place of the other one, suddenly my computer started booting OS9 with no trouble.
I suspect that the CPUP resource has been altered to include code or CPU model #s not in the original.
Give it a shot. It can't hurt anything and you can always put the original back.
The only issue is: To send it intact, I've Stuffed it.
Since you can't boot OS9, you'll have to unstuff it in OSX and then drag it into your OS9 System.
This will require you to have a UnStuffit-capable app in OSX - Mac Archive Utility will NOT work, so…
I'm including a copy of "The Unarchiver" which
will work in OSX.
I had to use the built-in Mac Archiver to make it small enough to attach here.
You can also download it at:
https://the-unarchiver.en.softonic.com/macUnzip the app, use it to Unstuff the file then replace the one that's in OS9 System Folder/Extensions.
If the System has a Multiprocessing Folder with another "CPU Plugins" file in it, remove that too.
Put the original(s) anywhere outside of the System folder temporarily.
Then…
Cross your fingers and toes and give it a shot.