As for burn-in it settles after about 2-3 days. Not dramatically, though. I don't even know what improves dramatically with burn in, so let's not use hyperbolas when talking about these things. It'll never have the punch of a 300W amp with linear power supply, so that's that.
I have a couple of questions;
Can you bring the AHB2 to clipping easily?
I'm asking because I'm considering this:
M22 needs 1.73V at input for 300W/8Ohms. and AHB2 needs 2V for 100W/8Ohms. Altogether, the NAD is 6 dB louder for the same input voltage.
Are you cranking up the volume 6dB higher for AHB2 when doing comparison?
When comparing amps of different power, many things that seem to be advantage are lost after the volume is set so both play equally loud, and not louder before the weaker one starts to clip.