Hello James, sorry to state the obvious but are you sure their isn't a issue with the batteries your using? I know we can eliminate the bulb being the problem, as these bulbs either work properly or they don't, and this one obviously works.
To be honest I am a little perplexed and confused, as you say the "M6 is just slightly brighter than my M91W". That would indicate to me that the M6 is functioning correctly, as the M91W is conservatively rated at 450 OTF lumens and your saying the M6, with it's 500 lumen rating, is slightly brighter, which is exactly what the results should be when comparing them.
But at the same time your saying the M6 is dimmer than your C2 with MN60, but then you go on to say that the M6 is still brighter than the M91W?. The M6 is dimmer than the C2/MN60 but still brighter than the M91W? I don't understand how the M6 can be brighter than the 450 lumen M91w but at the same time be dimmer than the C2/MN60? That seems contradictory to me. Ok, now I'm getting confused, just trying to understand the comparesons you've made here.
Sometimes comparing the output of different lights can be confusing, as the beams and color temperature can vary greatly, and that can make it difficult. Also the first photo you posted looks very much like weak cells to me, the yellowing is typical of what happens when the cells are weak. That MN21 really sucks the life out of the cells, and runtime is painfully short.