Please see:
http://forum.tvfool.com/showthread.php?t=13646
Start with the correct understanding of what a preamp is for...
A preamp is only able to overcome the losses in cable, splitters and some of the noise produced inside the tuner. A preamp is not able to 'pull' signal from the coax, antenna or air. So, only the losses on the output side of the amplifier can be overcome... losses and noise that happen before the amplifier can not be corrected by the amplifier.
Then, understand that all amplifiers generate noise and distort the signal. Good amplifiers add little noise and do not produce much distortion. Also, all amplifiers have a limit to how strong the signal can be at the input. If you exceed the input capability of the amplifier, it will be overloaded, causing a great deal of distortion and noise... the output will be difficult or impossible to be tuned and received by the TV.
So, you need a reliable signal at the antenna before any amplifier is added. In theory, you should be able to install the antenna, then connect a tuner directly to it and have stable, reliable reception with no amplifier.
You can then add up the loss in cables and splitters. RG-6 should have no more than 6 dB loss per 100'. Splitters should be labeled or otherwise have their insertion loss provided. You then need to look at the net NM (noise margin) available to the tuner at the end of the cable run.
In a perfect world, every signal would arrive at the tuner with a net NM between +40 & +50 dB... in practice, a +5 might be enough to be reliable, with few or no dropouts. I try to design for at least +10 to +20 NM. I'm reluctant to tell someone to spend money and time if those goals can't be met. However, the ultimate goal here is reliable reception, not some signal meter indication or answer on a spreadsheet.
Here are some examples of NM calculation and a simple spreadsheet:
http://forum.tvfool.com/showthread.php?t=109
So, you need to know how many dB of gain you really need. An amplifier with a lot of extra gain may not hurt... but it doesn't help either. Many amplifiers with high gain will be easier to overload.
Next, look for low noise specifications. The common term to look for is noise figure (NF). The lower, the better, 3 dB and lower is often the minimum acceptable value.
Finally, be aware of the amplifier's intended application, 'deep-fringe' and 'weak signal' amplifiers are going to be poor choices if you have one or more strong signals, including FM stations.
