You don't need to get the most powerful ones, but look for gain figures in the 12dB to 18dB range. Those 55% numbers you are seeing will go way up.
I am looking for a pre-amp as well. I get CBS(WCAX) at 70% with my active color antenna/amplifier. I was thinking maybe a good quality pre-amp with my antenna might give me better results. Why do you recommend 12-18dB range? And can I assume that the higher the dB the more powerfull the pre-amp?