Low signal levels

.I had a tech come out and check the incoming signal level (5db) and he indicated that was normal. He gave me a better grade cable to route around the room with the cable modem and it brought up the signal by 1.5db. Now my cable modem is reporting -13.5db which is acceptable but it is connected to the first cable splitter coming into the house. My problem is I have 3 tv connections requiring additional splitters and the bedroom TV gets a bad digital signalson several channels. I have a 12db cable amplifier which is rated +12db for 50-900MHz. Is this bandwidth enough for Digital cable? I tied adding it directly in the cable modem coax line to try it out and the modem showed a great signal but the internet was dead for some reason (no upstream path??).. Would this be Ok on just the TV portion of the system? House was wired in 1998 so the cabling shouldn't be too bad.