I have a problem with my cable tv. A feed comes in from the cable company and it is split behind a wall where I cannot access it. Then it goes to the tv in the basement. Recently I added another splitter to put cable upstairs and a cable run. I tried adding a 10db amp before the 2nd split, but it didn't really help much. I drew a picture in paint to illustrate.
https://netfiles.uiuc.edu/arawlin2/www/wiring.GIF

Can you get to the cable at a point before the first (hidden) splitter? That might be the best spot to amplify the signal.

Also, be aware that you need a bi-directional amp, since you have a Cable Modem. You'd also need it for any PPV and such, since the Cable boxes talk back to the Cable headend.

I have a problem with my cable tv. A feed comes in from the cable company and it is split behind a wall where I cannot access it. Then it goes to the tv in the basement. Recently I added another splitter to put cable upstairs and a cable run. I tried adding a 10db amp before the 2nd split, but it didn't really help much. I drew a picture in paint to illustrate.
https://netfiles.uiuc.edu/arawlin2/www/wiring.GIF
What are the problems you are having with your cable?

All of your problems could be caused by that splitter. You must find a way to access it.

With those footages and that number of outlets, you absolutely do not need any kind of amplifier.