All sorting functions have now been centralized into the "Mix" rules (formerly called "Limits"). Now you can choose a secondary and even a tertiary sorting criteria, if you wish.
In the above example all songs are first ordered by Artist (in ascending order). Then within the same artist the songs get sorted by rating (in descending order), then within the same rating they get sorted by their play count (again descending). In other words: Highest play count per highest rating first for each artist in the list.
This should satisfy all your sorting needs.
Note: Albums are always sorted by track # (unless you specify another sorting criteria).
Deleting the first sorting will give you a different random mix with each search.