I have been running full range for many years. Don't limit the lower crossover at all. You will get a much better blend and it will be more seamless running full range. That is the idea of a single driver???
The sub can compliment the system and you will only have to play with the upper cross on the sub and how hard the cut off is.
Finally, if you are finding that your single driver is distorting because you like lots of bass or lots of volume then that is another story. Most single drivers will fill a room but not rock it.