What kind of power on the sub?What style box?
Where will it be placed?
In what vehicle?
All of those things make a difference in what you need for mids and highs. At first blush I would say that for sure you need an amp on the mids and highs but if your car is a late model with a nice factory system, the amp may be more important than replacing the factory speakers.
And if you plan to put the new sub in a sealed box, in a trunk, with 500 watts on it.... no need to change anything about the front stage. It'll keep up with that, unless it's a '62 Plymouth Savoy with one speaker in the dash. ;-)