Consider that the runtime of a flashlight is generally based on two things:
1) the total wattage-hours in all of its batteries.
2) How much wattage the light uses in a particular mode.
For 1, you want batteries with the highest energy density. The more the merrier as obviously a 500mAh AAA that runs at 1.2 volts doesn't have much charge.
A 2xAA battery will do a lot better. An eneloop runs at 1.2 volts but has 2000mAh each.
CR123s are pretty good too. A rechargable (RCR123) has 4.2 volts and 750mAh which makes it about on par with an eneloop (remember, wattage-hour = voltage times amp*hour).
An 18650 is a solid choice, about equal to 2 RCR123s. Most 18650 lights are more high powered though, and they are less efficient at a low mode.
For 2, its just a matter of finding a light that gives the desired lumens. Most lights are about equally efficient. There isnt enough variation I don;'t think to really matter much.
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You don't HAVE to go with DX. It takes forever to get your light, and quality control issues abound. There are several good vendors that you can find at our Dealer's Corner that have reasonably priced lights.
I think your best bet is 'itp' brand. Their SC2 light uses two CR123s respectively. They CAN USE RCR123s too!
Its 40 bucks (minus potential discount) at most places, and the cool feature there is its infinite brightness adjustment!
For someone wanting a true 'low', this is a fantastic feature to fine-tune exactly how much brightness you want. It is tons better than just having to go with whatever 'low' mode you get with the majority of flashlights.
It has the capability of spewing out a lot of light, which is nice to have! But you can easily use it as a low-only light. It is rated at 37 hours of 8 lumens! That should do nicely.
Plus, it will be of good quality and you shouldn't have any hassle at all.
I'd check out goinggear where Ive bought my lights and look for itp SC2 2xCR123. See if you like the style, etc. Then do some comparative shopping. They give free shipping for a 50 dollar order if you want a small camp-gear piece. 10% discount with their coupon code (found at dealers corner forum here)
Romisen has some cheaper options too, but they don't have enough of a low to fit your needs. 4sevens has great lights, but they get into the Expensive category fast!