It feels strange because the 880 and the 80 look like the same speaker but they are not. Remember that the 80 is passive while the 880 is partially active. And it has rear drivers. Using the DSP, Axiom was not only able to combine the radiation patterns of the front and rear driver arrays to lend to a smoother sound power response, it was also able to correct for resonances to some extent down low.
That curve down low has nothing to do with the HP driver. The HP driver only matters when you start pushing the speaker into higher SPLs like I described previously.
If I had to choose between the 880 and 660, knowing what I know now, I'd choose the 660. I say that because in my 4200 cu ft room, I can play the M5s as loud as I need to with a sub and no distortion. The 660 will play much louder before distortion kicks in.
I know I sound like a wise guy when I say this but I'd bet you would not return the 660s. They will sound very good!
