Personally speaking, I think there's a great misunderstanding about preamps. If you are in an urban area, or your scanner is prone to hearing junk in places it's not supposed to be, a preamp is likely to make things worse not better. In fact, due to the way a FM signal is detected, you may even experience a loss of signal.
Preamps should be used in an area where signal strengths are rather low, or in bands where signals may not necessarily be very high, such as in the milair field.
A very good rule of thumb is to improve your antenna and coax situation first, before considering a preamp 73s Mike