The previous owner had disavowed 400+ backlinks, but I know that some of them were very good and high quality. I un-disavowed them. (Avowed?) I don't know what they were thinking, but how do I determine and know if backlinks are hurting my SEO Rankings? Google's disavow tool says:
We recommend that you only disavow backlinks if you believe that there are a considerable number of spammy, artificial, or low-quality links pointing to your site, and if you are confident that the links are causing issues for you.
However, it doesn't say how you can be confident the link is causing an issue.