[paulypaul]

If you like mids the RE-ZEROs really shine.
The M2s are nice fun iems which are warmer and have more bass than the RE0s, but they lack a bit of detail compared to the RE0s and RE-ZEROs.
Both the RE0 and RE-ZEROs respond quite well with bass boost.
The ZEROs and the M2s are my 2 favourite IEMs at this time.

 

[hctu]

I purchased the RE-0's and as others have said - you really need an amp to make it sound more a bit more "full" in my opinion.  I also had the CX300 for comparison.  Although the level of details in the mids and highs of the RE-0's were fantastic, I just find that the lack of bass makes this less enjoyable on some music.  This is VERY noticeable especially if you are changing earphones around for comparisons.  I can't help thinking that for this price, I probably could have done a whole lot better!