Do you want more mid-range in your rear-fill or just a solid/warm mid-bass presence? You could probably get a very nice sound with cross-overs set from 100 - 3 KHz, but that will give you some mid-range too. That may be nice to have the guitar (lead, rhythm, and bass) and keyboards having some presence around you, but vox still being kept up front.

There are of course, a plethora of settings to choose from. I would start at those levels and then play around until your ears allow you to smile.

UB

you're not gonna hear it directly anyway, especially at the power they recieve (should be alot lower than the fronts),

as for 2500 or 5000hz cut off, i wouldnt bother too much, becoz the speakers /mids will roll off naturally by it self