I can speak for the sources too, they are humans and they get tired of reading through bucketloads of cesspool.
One idea that came to me:
Apart from merit sources, another group needs to be created like "Merit Pointers" - these are users who help the merit sources reach the posts that they feel need merit. We already have people posting their list of merit worthy posts on threads created for similar purpose, but a button to mark these posts seems to me as a forward step.
If you observe, you will see posts that have 20 merits, 10 merits, 8 merits, 5 merits, 3, 2, even 1 merits in just one post but get merited by different users, both merit source and non-merit source. That is not because the post is not good, but some people who give the post lower merits see the post as not quality enough and those who give the post high merits see the post to be high quality.
So that is to say, you might not like what I like, and I might not like what you like.