I agree that an amplifier is bad for your situation. I live in Akron Ohio, just a mile to two miles from 4 TV stations, and the amplifier reduces my signal when I turn the gain knob up. Don't use an amplifier.
Judging by the TV Fool results, your signal level is shown as really good for WCVB. A 200' cable seems really long, but I don't think it would hurt on WCVB since the noise margin is already 57.3 dB. The estimated cable loss is around 5 dB per 100' of RG-6 cable. Is there anything blocking the antenna? If it's directly behind a tree, UHF signals get reduced 90%. Distant object like houses and trees aren't as bad since signals can diffract around them, but nearby objects interfere a lot. Could multipath be an issue? Maybe your just in a weak spot for WCVB but not the other channels. Try different antenna positions, move it up and down the pole.