Looks like I'm a little late to this thread, but for what it's worth, here's my 2 cents.
I run and manage writers for a living, and just like in any job, passion for the task waxes and wanes. An in-house editor is a BRILLIANT idea, one that I wish most clients would take to heart. Depending on the volume of content that you produce though, you may need more than one.
However, if your contractors writing is becoming dull and generic, it is likely because they are struggling to find new things to say about what you are hiring them to write. No matter what the subject is, unless you are asking them to write whatever they want, their content will become stale.
The key to keeping a writer invigorated and passionate is to give them some new subject to dive into. If they are in-house writers, maybe incentivize them for articles and posts which go viral. Help them with research by giving them new topic ideas that they haven't considered because they've gotten into a "comfortable rut".
Bottom line - if you want your contractors to stay engaged, you have to engage them. If their writing is dull, it's because they are bored. Either get new, fresh writers, or get them fresh topics to write about.