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So, how do feel about these vs your CK90Pro and did you do the foam removal mod on those?
Interesting comparison, because the MC5 and the CK90Pro are so different in character (and price). I did do the foam mod to lighten up the sound a little, but to be honest the amount of foam doesn't have as much of an impact as actual source selection for the CK90Pro - for instance, I used to get slight sibilance on my old iPhone 3G and Walkman S739 on the 90Pros that I thought was frequency uneveness, but using the 90Pro with a uDAC or my new iPhone 4 that sibilance disappears - so it may actually have been peaks in the frequency output caused by a weak amp distorting. On these sources I find I can remove the foam entirely and not get any sibilance.
In general though, the 90Pro is a much bassier sounding phone, and while the MC5 is a treble detail orientated, slightly forward IEM, the 90Pro is more neutral - though more on the darker side of neutral. It's darker than say, the RE252 for instance. Instrument separation and soundstage are more pronounced on the 90Pro than the MC3, but the 90Pros do not have that same sense of treble airiness. The other important difference in the sound, and the key signature of the 90Pro is its speed - the CK90Pro has such fast attack and decay times, meaning that bass is punchy and at the same time has no time to really "soak" in. So while this makes the CK90Pro my favourite IEM for electronic music, since it does pure tones and glitchy effects so well, it can actually sounds less realistic and transparent than the MC5 when playing natural instruments, particularly cellos which sound a bit plasticky.
It's hard to compare the realism of the two IEMs, because one sounds like how natural instruments would sound if you took away most of the feeling of airmovement from bass sounds, and the other sounds like how natural instruments would sound if you were trying very hard to replicate natural sound with no resonance.
In terms of comfort and build quality, the CK90Pro is more comfortable by a pretty big margin, especially because I have no trouble wearing the 90Pro over the ear whereas I can't with the MC5. Of course isolation is king on the Ety. Build quality, the cables are much nicer and less microphonic on the Ety. Whereas I used to think the build quality on the 90Pro was pretty good, lately the cable sheathing on my set has detached from the neck divider, so I can only give a reserved recommendation on those terms.
Sorry for the long winded explanation - I wouldn't be able to pick out exactly which one I like more, since the 90Pro is really very fun to listen to for modern studio recorded electronic music (which I listen to quite a lot) but the MC5 is nicer for acoustic, classical etc. I hope that helps!