That's because they are not getting a good seal on the M6. It should be IEM 101 or kindergarden that with a bad seal, no matter how bassy the IEM, the bass will run out and the IEM will sound overly bright. This is why the manufacturer includes several tips, single flanges, double flanges and triple flanges, so that folks may achieve proper seal no matter what ear shape.
Go for the V-Moda Vibes, great Bass, good mids and treble. Over 400+ positive reviews on Amazon. These are my old stand-bys when I am not listening to my Monster Turbines. In the $25.00 price range, they absolutely smash anything else in this category.
I found the M6 to be far to sibilant and lacked any type of Bass, but some people have had some sucess with these. I would check
the forums here though, there is even a current post describing the harsh treble and lack of bass, so I would not take a chance
on them for now.