1. The 880 goes lower and you can see it in the graph by the slope below 50Hz. Above the low end, they do look very similar. I don't see the 880 graph being smoother than the 660 in the high end.
2. I've said it before. The HP driver is for those who want linearity when they drive their speakers hard on the low end. Axiom has never quantified what this means. From my listening, if you want linearity below 80Hz at 100 dB SPL at 14 feet, get HP drivers. Or even better, get a sub or four.
BTW, those curves you see, that's at about 1 Watt input. What we really need to see is the HP and non-HP graphs at various higher power levels so we can compare.
3. Yes, they do.
