Will Parasound A21 damage my NHT Speakers?

Very doubtful that this amp will damage your speakers.

Most speaker damage comes from driving an under-powered amplifier too hard, and driving into it's 'clipping' stage, where power spikes can take out tweeters and other drivers. Very few speakers are damaged by too much 'clean' power.

In other words, your would have a LOT better chance of damaging your speakers by driving them with a 50 wpc amp.

Amplifiers don't kill speakers, people kill spealers.

Under powering can damage tweeters. Over powering can damage woofers.

I agree with the preceding responses. I would add that you should be sure to follow the proper turn-on and turn-off sequence, which is amplifier on last and off first. Some preamplifiers and other upstream components can generate wicked transients at turn-on or turn-off, which would not be healthy if amplified to 400+ watts (perhaps even clipped) and applied to your speakers.

After turning the amplifier off it would also be good practice to wait some seconds before turning off the preamp, since the amp may be able to continue to amplify for a short period of time after being turned off, using internally stored energy.

Regards,
-- Al