You are very much on the same wave link as me on this. I figure the 6.5 will run out just at the point where directionality matters so with 2 or 3 subs you would have a very nice low power home theater system.
The secondary reason for this is when I am camping out in the Rockies miles from a power source. Music is a must around the campfire so that is a big part of the reason for a TA2020 driving the speakers off of a spare 12V auto battery. If I want a sub up in the woods though, that is a little more challenging. How would you increase the efficiency of the 3Pi sub? I know, I know, 92 is great for a sub. I would also want to drive it with a digital amp to keep the power draw to a minimum - hopefully a tripath based one- still need to figure that out.