"Will Parasound A21 damage my NHT?"
Short answer, no.
Long answer, the ratings given for speaker watts max are over long use, at prolonged output. Basically, coils heat up, but take far more time than our ears can stand to do so.
What will damage your speakers, is too little power. When an amp distorts, or clips, on output, it sends a distorted signal to your speakers. Prolonged clipping can damage your speakers very quickly. Usually, tweeters go first.
Incidentally, I had a halo a21 with my NHT 2.5i's long ago, and they sounded great together. You'll love the parasound with your speakers.