Can you clairify the bit about large cones not handling anything? Because that seems back-ass-wards.

You can calculate at what frequency a speaker will begin beaming, 18's dont beam till around 200 which is still not that low.

When you can hear audible distortion you're at 10% THD+N typically...

which is about 10 times as much as you want.

Loudness curves neutralize themselves as you turn it up... they take care of themselves.