Since scanners are already pretty sensitive, a preamp should only be used to solve a particular problem. If the signals that you want to hear are already fully quieting the receiver, but just don't make enough "bars" on the s-meter, a preamp won't help you. If the signals are 'hissy', it might, depending on the preamp.
Here's a few things to keep in mind. It's probably over some readers heads, but here's the facts:
A preamp MUST have a lower noise figure specification than the receiver, or it won't actually be helping. If the preamp doesn't list it's noise figure, then don't buy it. Period. Your scanner noise figure is probably 4-6 db.
The only gain required is just enough for the preamp to overcome the noise figure of the receiver. No more. A 1 db NF preamp with 10 db gain is better than a 3 db NF preamp with 25 db gain. Lots better. The lower the noise figure, the better the preamp. Do not buy a 4 db NF, 25 db gain preamp and put it on your receiver and declare that "it works!" just because you have "more bars". That's not how it works. Base your success (or failure) on how well you're hearing weak signals with and without the preamp. Additional interference and intermod=failure, even if you're hearing weaker signals.
A preamp should always have a filter in front of it to avoid overload. This means your multiband scanner will be reduced to a single band. If you live in a really quiet area with no nearby cell sites or other transmitters, you might get away with a broadband preamp and no filter, but don't be surprised if that makes things worse.
There is a specification for how much overload a preamp can tolerate. It's called IP3. The higher the number the better.
