The best RF amp in the world won't increase range beyond what FM is capable of. Depending on location and power of the transmitter, height of antenna, and terrain in between, you might be lucky to get a decent FM signal more than 20 or 30 kms away. This can be improved with a better external aerial but FM is still limited by its intrinsic capabilities. The signal will bounce around between buildings and trees and hillsides, affecting its quality, and it will be limited by the curvature of the earth. New Zealand is not FM-friendly countryside. If you want consistent good reception here on the road, AM is a better choice. If you want quality, play a CD.