So I think part of the problem is just the lack of points of comparison... for reference, what kind of Monster Beats IEMs are we talking here? There's a number of them, aren't there?
I'm thinking you might want to look into something generally warmer and tending towards U or V-shaped for those listening volumes, with a laid back mid-range and probably some elevation in the mid-bass but the treble boost pushed out towards the upper treble, not so much upper mids / lower treble. I would tend to think that anything described as somewhere closer to balanced or slightly bright is probably going to tend towards harsh
and A-shaped at those volumes.
EDIT: NOPE, that A-shaped bit is just downright wrong, balanced goes to V-shaped at elevated volumes
, but harsh still applies, and I think the rest might be worth considering - one man's warm, smooth, intimate and romantic mids can be another man's recessed to oblivion and completely absent any detail. I feel like you want more lower mids and less upper mids, but I also feel like I should stop rambling as you're really far outside of my comfort zone and I just feel compelled to respond as one of the only other people with a pair of NC50s in hand.
As for where to look, the other Chi-Fi thread tends to have more information about the next price category up, but you might want to jump outside the Chi-Fi threads too... there's plenty of great V-shaped sounding products out there out there as it tends to be a very consumer friendly sound shape.
EDIT AGAIN: I feel the need to clean up my ramblings a bit... by adding to them, yeah, that's surely helping
but perceptions of sibilance, fatigue, brightness and forwardness of the midrange tend to reside in the same general region (upper mids / lower treble), so reigning in fatigue and adding enough warmth to the mid range for you without putting it completely in your face might push the mids back far enough for someone else to call it V-shaped. That's where that jumbled line of thought was going.