I would go 22 max in the back, the XX has a high ride height to begin with.
Mud lights hook up good for the first few rides, IMO they burn down too fast, i rather go with swamp lights friend had them, very mean looking tire, deep lug, great for trail riding and mud ( i would guess, we dont mud with our 700s), not too good for loose coal if your into step hill climbing with em.
Here is what they did look like on his.
On a side note, i just put the Cutters on the front of mine, to me they work as well as the GNCCs i had if not better, well worth the 60 bucks each.