Can you post the reference video out of Media Composer as well to compare?
Things to make sure:
Avid timeline matches same aspect ratio as one in DaVinci. Even better if you can match same resolution, so if delivering 1080, make a 1080 project.
All sources are AMA linked and kept at their original aspect ratio for editing.
If the aspect ratios match, then you are just as well off using the 3DBlend or resize to create the crop as Resolve will give the same results.
Michael