Hi Rock,
I agree with John K and (mostly) with Mojo. Given your playback levels and the size of the room, your Nad's 80-watts per channel should be quite capable of driving the M22s to clean loud levels without distortion.
Try bypassing the Oppo and using the Nad's own preamp/amp to drive the NAD and listen for any distortion. If the Nad has bass/treble controls, do NOT use any bass boost, which will increase power demands from the Nad amp significantly.
By the way, there are NO audible differences between modern DAC converters. Measurable differences may exist but at levels of -80 dB or greater. Those are totally inaudible to human hearing with music programming.
As to misleading statements, I do believe in buying as much power as you can afford, to eliminate the possibility of driving an under-powered amp into clipping distortion. I do not endorse the use of tiny tube amplifiers with any speakers. Those tube amps routinely generate distortion well into the audible realm (1% to 10% or more), and have many other liabilities.
I agree the dB meter apps should be calibrated. The one I use was off by almost 10 dB until it was calibrated. However, the levels you measured seem quite accurate for comfortable listening levels in a domestic environment.
Alan
