Yeah, any issues you may have with FREQ response, you can use the eq on your portable. The cmoy is going to mate really well with these, and they are a good headphone, accurate, and built like a tank. The earpads aren't the flakey kind Sony uses, or senn in their closed phones. They need to be broken in, burnt in longer than other headphones to take a little bit of the sizzle out of the top end, but if you are going straight for lossless and very good lossy, as in lame 320k, the hf issue shouldn't be a bad thing, but rather something that allows the phones to open up and give you a bit better performance. I see them as good for IDM, russian minimal, dub minimal, etc, but also acid jazz, then I see them forclassical music. I use them for these genres, as I don't see them as a "rock phone", but that is what is great about this hobby, I own enough phones to not have to worry about what i am going to do when I just want to rock out.
The most expensive component in my system is my amp, but as a speakers audiophile, I was extremely frugal, but it is easy to spend a bit too much with headphones because "just another 100" actually gets you performance you can hear. I didn't always hear the difference between a couple DACs I have, or certain tubes, but on a headphone system, you can't help but hear it and want to make that sound your own. That tosses the puist ideal out the window, but it also could be said it makes the purist ideal more personal. I do not think rolling tubes is coloring the music in a way that is not true to the recording, I see the various harmonic adjustments, or a little EQ on the mixer, as shaping the output of the headphones to the performance of MY EARS. Grado is a love em or hate em brand, but the folks in the camp that enjoy them, don't think about lying and saying Grados have a flat freq. response and no individual character, no, they feel the unique properties of their headphones brings them closer to the music in more than just a physical way, but an emotional and intimate way. These are accurate and true to the recording, and ask for some amplification, but not so much that a simple protable amp cannot easily supply. The accuracy does what a lot of phones cannot, which is re-create the complex and rich harmonics of electronic music, especially IDM and other experimental types (this is why they would work as a rap phone), as well as the extreme complexity of a symphony.
I hated them until I learned this, as well, when I dropped the speakers mentality and considered how personal headphone listening can be, so WHY NOT use a bit of EQ to bring the sound closer to what your own ears should be given for the ultimate enjoyment of whatever one is listening to.
If you use them with a PC as well, get the electri-q plugin, it is a vst, but also the best winamp dsp ever made. Then try ozone, and then try headplug crossfeed via DSP. Electr-Q alone can consume hours of playing around, taking a headphone that is meant for a certain kindif sound, and adjusting i for your ears. Speakers, probably, need the purist asthetic, because a 2-way or more set of speakers are designed to playback a naked signal. Work is done to solve all the problems with phase, or room modes, bass, port tuning, and on and on, but headphones are a single driver per ear, so close that all the things that get lost in the air on a speakers setup makes it to your ears. Already you are hearing something that is of a different paradigm than speakers.
In short, headphones are intimate, and so it makes sense to dial them in to your liking. Small things matter! Audiophiles in the traditional arena of speakers obsess over small things, but even an average headphone rig is more than enough to make these small things actually matter, it is no longer a question or doubt. You hear it, and it is satisfying to listen this way, and satisfying hearing the character that had been so elusive, be right there about your head.
The AT m50 is also a good phone for the cmoy, and I love my set, but I agree, the Shures have a professional design that just overflows with the concept of "I am a serious music lover, not some top 40 zombie, I like rap, but you aint going to hear THIS on the radio ;)"