I remember when I had the NHT SuperZero bookshelf speakers. For 2.1 stereo music, I never heard distortion even at 90dB volume. But for movies 5.1, sometimes I would hear the "crackle" sound on dialogues at high volume.
When I changed my NHT system to Def Tech, the center was the CLR3000, which was much more dynamic than the little NHT SuperZero. Same 50WPC HK AVR. Same movies. I don't think I ever heard those crackle noise again. I think the minimum impedance on the CLR3000 was about 3 ohms.
So I usually don't blame the amp first. I usually blame the source or the speakers first.
NHT makes accurate speakers. So I just think the problem was the lack of dynamics in those little speakers to handle certain compressed soundtracks at high volume.