I'm often a supporter of listening beyond the specification, but on this occasion your experiences and the specs seem to meet and make sense.
The AVR 2105 will certainly have been delivering more clean power to the Dyns than either the Marantz or NAD are capable of, and despite Dynaudio speakers offering a fairly linear impedance, it is still low and at only 86db they aren't particularly sensitive either. The fact that you didn't experience the same issues when using the Denon does rather bear this out.
One very affordable amp that might be worth a try is the Yamaha A-S500. This amp delivers plenty of power, is very clean sounding and has a setting specifically for driving lower impedances. You would have to audition one as you might not like the sound, but it certainly won't suffer in terms of power delivery like many other budget models.
As I've said before, the Cambridge Audio 650A is well worth a listen too. Very capable amp indeed with lots of clean power on tap, supported by a big and high quality toroidal transformer.
If you like the NAD sound but just want things to open up a bit you could do much worse than looking at second hand models like the C352, C355BEE, or the C370.
Beware, though, it isn't just about power rating but also about current delivery. Furthermore those Dyns will be quite revealing and to get the best out of them you might well need to up the ante. Might be worth saving for something like the Audiolab 8200A or the Naim Nait 5i, but again an audition is essential.
Of course, the other option is just to change the speakers to something more suitable for the sort of amplification you are considering.