I can't see how you can look at that article and say the sound is identical at all output levels.
Well the SPL differences between the 550 and 650 show up around 400W load, but by this point, they're effectively the same: At 36 or 38 dBA/1m, they're both loud.
Seriously, there's really not much to differentiate between 40 and 43 dBA at full load, either. I've listened to both; in real use, I have a hard time believing anyone could hear the differenes between these models.
If there was a 2-3 dBA difference at 20~30 dBA/1m, then it would be much more significant, but at much higher levels, the same measured difference does not sound
the same, subjectively. It must have something to do with how close to audibility threshold the noises are on the low end, and how much beyond your "acceptably quiet" tolerance level is at the high end.
Perhaps for someone else the difference between 40 and 43 dBA might be significant; it is not to me. I would simply not tolerate either if I could control it -- as in my own PC.