I also like the Seoul P4 a lot. At 1000ma drive current, Vf is probably about 3.5V. This allows you to run your light in regulation with a single Li-ion cell, and even get reasonable direct drive output from a single Li primary. With the right driver, you can run your light with a wide range of battery options, such as 1xLi-ion, 2xLi-ion, 3xLi-ion, 2x123A, 3x123A, and 4x123A.
You can also configure a P4-based AW tower for a "low voltage" option if you use a boost driver fed by 2xNiMH, though this is probably irrelevant for an M6. Can be relevant, though, if you have a FiveMega 2xC cell body, and use a Classic to Millennium adapter to fit a KT4 head (or directly use a KT1/2 head).