Where's the best place to put a cable signal amplifier?

In the TRASH.

If you have a crappy signal, call the cable company and tell them to fix it.

You could have TOO MUCH signal and it will mess up your picture.

Most picture problems are not from the amount of signal, but are from the
amount of stray interference getting into the cable at bad crimps and
damaged cables.

A cable amp is about the worst thing you can do if you've got a bad TV
picture.