Ken, good question with respect to power density of the 2D vs 8AA. Of course, the answer is "it depends".
If all you're doing is running a single Lux1 at spec, there's absolutely no reason to use AA's. If you're serious about running 4 Lux at or above spec, then you could make a case for 8 AA's. This would make a great expirement actually.
I haven't tested any of this, so I may be a bit off, but here's my take...
Figure that 2 "D" cells will need to provide
at least 4 amps to drive four Lux3 at spec (12 watts total). That same 12 watt requirement can be meet with 8 AA's and only a 1 amp drain. How long will a pair of 10,000mAH NiMH "D" cells run at a 4+ amp drain? How about the 2,500mAH AAs at a 1+ amp drain? How efficient will the boost circuit(s) be in the 2D scenario? You'll need at least two, most likely four. In the 8AA scenario you'll need one buck circuit, two at the most, and they tend to be more efficient than boost circuits.
Maybe someone out there has some real life data they can share?
As VTWim mentioned, you could always go the 3C route and direct-drive two or three Lux3. It would be very bright (although it's unregulated and you'd burn through alkaline "C" cells). You could also try to direct drive two or three "J" or even better "H" binned Lux 3's using 3 C NiMH.
Jon