I tried with 3 apps to measure my normal playback level. average is below 80, peak at 85, not exceeding 90. Actually I quickly put it to full volume and it just reached 90 and couldn't reach higher. Since the app hasn't been calibrated so the result might not be accurate. I measured the level of a piano and also got the same range (average around 78, peak at 85)
Most of the time I feel comfortable to enjoy the music without any distortion. Just when I increase the volume out my comfort zone or the music gets too loud, then I feel the hi pitch sound is getting too high and narrow.
I would not trust the smartphone mic or a smartphone app for this application.
Like Alan said you may be saturating the NAD's input with your Oppo.
Try driving just a single speaker. If you are hearing distortion out of only a single channel, it could be the channel or the speaker.
