Sounds to me that you have a problem with the o-ring inside the faucet...either damaged or misaligned. See items #2 & 3 here: http://www.perlick.com/store/webcata...ctno=1&grpno=4
Also, check the condition of the bearing cup (item #6) for damage.
It will have nothing to do with the nut closest to the handle. But, if you tightened down the lower nut (item #7) too tight, you may have damaged the bearing cup or the o-ring. Those faucets should seal with just a couple drips without any pressure to speak of...almost as if when you close it, the beer pressure pushes the handle tight against the oring. That bearing nut (#7) does not need to be super tight, just snug (a strong finger tightness- NO WRENCH!).