1. The 67000 is immediately audible as smoother, more natural sounding. Most folks loved the 66000 at my place, and had no problem in this area. But I like the added smoothness of the 67000.
2. The 67000 is clearer in the midrange. Lower level details are more audible.
3. The 67000 bass is more detailed, more linear.
4. The 67000 has all new crossovers, new wiring, new level controls, new woofers, and a new crossover capacitor bias system that does not use batteries.
The mid and hf drivers remain the same.
The 66000 is a great speaker that does not shout, honk, quack, sound like cupped hands, or any of the other ridiculous generalizations folks make about horns. But I think the 67000 is a nice refinement above the 66000.
I am not a JBL junkie or groupie, I never owned JBLs before the Everest, I have no financial other interest in the company, and I own other speakers too. But the Everests are great speakers.