Pre-Amplifiers are mistakenly used with the rational that it will bring in Stations that weren´t there before... It won´t, and what´s worse, using the same antenna and pre-amp with FM and TV stations close by (0-30 miles) may overdrive the amp and actually impede your ability to provide goods signals to your TVs. If you desire to use a pre-amplifier, don´t use a high gain when you have local stations, get one with a FM trap to keep FM radio stations from over-driving the amp and remember that today´s Digital Signals don´t need to be high to get a picture. Some Pre-amps come Band Selective as well, realizing that the difference in antenna UHF/VHF gain means that one band needs to be amplified as opposed to the other. Use them to maintain Signal level to noise at the origin (your antenna) so that low signal levels from far away stations are not lost in a longer than average cable run to the location in the house where the signals would be further distributed to the TVs desired.
Amplifiers come with various gain numbers. Old Analog signals needed a high Carrier to Noise to get a high Signal to Noise Ratio of the Picture on your TV set, it was a one to one relationship and more was sometimes better. Digital Signals are lower in amplitude in comparison to Analog (a benefit) and that commensurately, the threshold at the TV is lower for a perfectly good picture. Very little in between, you either get a good picture or you don´t. Snow in the old analog pictures has been replaced by "Tiling" or little blocks within the picture not keeping up with the scene changes in today´s Digital TV viewing. Tiling is therefore today´s Digital Snow. It means (more often than not) that you are not far enough above the threshold of signal level into your TV to prevent little hiccups, drop-outs and interferences in signal from interrupting the Stream of Data. Remember that lots of things can happen from the Station on the way to your TV set!
The gain of a Line Amplifier should be selected to zero out the loss that follows it. In other words, if you have fifty feet of cable and a two-way splitter, then a 10 db amplifier will be fine. Conversely, if you have an eight-way splitter and hundreds of feet of cable, you will need an amplifier with a higher gain. Just as in the case of a Pre-Amplifier, however, bigger is not always better.