I don't want misinformation out there. To further clarify, 66000 are great and I would not try to dampen enthusiasm over them. However, having had 66000 and 67000 same place, same equipment, A then B, it is a fact 67000 is not one of those small refinement upgrades. Room treatment would not make this difference. Direct sound is direct sound no matter what treatment you do.
One thing to keep in mind is that the 66000 can be forward, and depending on the amp, can be VERY forward and prominent in the highs. Some solid state amps will drive you out of the room. I found tubes worked much better.
The 67000 are a lot more forgiving, and solid state amps work much better now. And tubes still work great.
So, if you go 66000 be prepared to possibly need to find the amp that pleases you. Luckily, a stiff 50 to 100 watt tube amp can do the trick. As for the highish looking efficiency, don't let that fool you, no 5w or 10w SET is going to get the woofers going to the depths of what the speaker is capable of.