One excellent example of such a beast: The Petzl Duo ($65; www.petzl.com), which has three LED bulbsbright enough for probably 75 percent of the things you need it forplus a very bright halogen bulb. Run the LEDs on their own and get 60 hours out of your four AA batteries. With the halogen alone, three hours.
Of course, the wild card is the temperature. The Duo's battery pack is attached to the head strap, meaning it can be exposed to the cold, diminishing battery life (though the Duo can automatically switch to its low-battery mode to conserve juice; I assume that's what you mean by a "voltage regulator"). Generally, it would have to be in the teens or colder for that to make a real difference, as even a head-mounted battery pack picks up body heat, especially if you have a hood or hat over it.
Additionally, there are some lights designed specifically for cold-weather use. An example: Black Diamond's Polar Star ($45; www.bdel.com), which has one halogen and one LED bulb. Its battery pack can be detached from the head strap and stuffed into a pocket or under a jacketa big help in very cold conditions. Petzl's new Myobelt 5 has a similar feature, plus five LEDs and a xenon-halogen bulb. Very nice, but steepish price-wise at $75.
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