Will Parasound A21 damage my NHT?

"Will Parasound A21 damage my NHT?"

No

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.

Couger: As above, NO. For comparison, I had the A21 pushing NHT Classic 3s (bookshelf). no problem whatsoever.

Good luck.

Also keep in mind that the exact position of your volume knob does not always play back at the same volume or power output.   If you are playing an older CD, full volume might be at 2 o'clock, but with a newer louder CD, full volume playback, the volume knob might be at 11 or 12 o'clock.   
I've always owned big 200 to 300 wpc amps, regardless of the speakers I'm using.... from small 6.5" 2-ways to big 12" woofer 3-ways.  You just need to use your ears, and good judgment on not pumping too much low frequency's through small woofers.  

I know parasound is 400w into 4 ohms. I usually listen to my speakers at 11 o’clock. How high can I go if I play loud before my speakers get damaged? I know nothing about amplifiers. Thanks.

Couger you been a member for over 10 years you should know, the amp is perfect for you, as for your statement above.
Forget the volume control position, your ears will tell you when things are too loud, they may bleed, thing will start to sound strained, and if you go any louder, you’ll may hear an almighty crack as Armageddon shows itself. And that will be the sound of your bass driver hitting the end stop or the amp going Chernobyl.

Cheers George