I'm going to guess that you talking about the TIVO 'DVR Diagnostics' screen. And if so, that is NOT dB levels. That is just a reference scale to go by = 0 is no signal at all, 100 is max.
TIVO recommends you have between an 80-99 signal on that meter. Below 50 and things start to screw up. 15 certainly would cause problems. And even 30 should be causing some problems, maybe haven't noticed yet.
Also on that screen, that is very important, is SNR. Should be at least 23dB (yes, that is shown in dB).

Sounds like you do need an amp (although other things could still be an issue as well). How much gain you actually need on the amp, would be determined by a digital meter - to measure actual signal levels.
Or... if you could find an adjustable amp. Start at the lowest setting and turn it up, until the TIVO signal bar is at 99 (or the highest number it will go at full).

I have used a CE Labs 6001 with good results. It has 20dB of gain and is adjustable down to 0dB gain. Maybe discontinued now thou.