You must be able to receive the channels at the point of entry to the premises, otherwise your amplifying noise. If a cable modem is involved more things have to be taken into account.
What does your distribution look like from the cable demarkation to where you could amplify, splitters/taps, wire length and the type of wire (RG59 or RG6)?
Can you put the cable modem shortly after the point of entry if you have one?
Go to the demark point with your TV and see if the channels are good?
Report back
BTW: Im amplifying about 10 TVs from a rooftop antenna and I use taps, not splitters. One of my runs has to be changed from twinlead to coax. In another case, the blank plate and holes in the studs are in place because of painting. Just have to run wire.