A discone with a preamp. Yuk. I can't imagine how noisy that would be, not to mention the extreme amount of intermod type of interference.
If you need an antenna to cover a wide band for scanning, consider a scantenna or similar antenna. RG-6 grade coaxial cable will work fine. Be sure to seal the connectors.
You do not need to use a preamp for wideband receive. A preamp will amplify noise and increase intermod interference. I'm sure you will not have any benefit to using a preamp unless you are way out in the boondocks, and there is not a pager or broadcast transmitter within 100 miles.
The number one way to increase the range of your antenna/receiver is to increase the height of your antenna.