The maximum voltage your Malkoff can handle is 5.5 volts. Using 2 D-cells, you're getting 2.4-3 volts, which puts 600 mA to the emitter. If you use a single Li-Ion cell, you'll get ~4 volts, and ~900mA so it will be brighter.
Either of the AA adaptors you describe will exceed the maximum, unless you use lots of dummy cells. 3x alkaline or 4x NiMH cells is the maximum.