sakso,
Your comparison of the 9 watt 300b amp and the MBL 300 watt amp is consistent with findings from other posters earlier in this thread. It drives home the unique design/intention of the very special driver used. An ultra controlled driver cone doesn’t need additional damping (DF) control from the driving amplifier.
As you and others have demonstrated excess amplifier DF will deteriorate the sound quality of the Nenuphar. Your powerful MBL amp very likely has a relatively high DF (and low output impedance). These traits are no doubt desirable for some (perhaps many) speakers. The Nenuphar is an entirely different approach. Given the superb results of the AC Genus/Nenuphar pairing one could assume the Genus amp has very little (or maybe zero) NFB and thus a low DF and moderately high output impedance. Ideal for the Nenuphar..
Charles