Not quite apples and pears. The K1k does have an electrostaticky vibe to it. It's one of the fastest dynamic drivers I've ever heard, is fairly flat in terms of tonal balance (but not completely flat), and scales absolutely incredibly well with improvements in the signal path. It does have a resemblance to the SR-404 in sound signature, but just that - a resemblance. It matches, or even surpasses, the SR-404 in soundstage size (both are huge but the K1k has more depth and significantly better imaging, while the SR-404 tends to sound rather diffuse with blurry images). The K1k also has a better instrument tone - the SR-404's upper midrange peak really destroys its tonality to my ears. The K1k is tipped up in the highs, but fairly uniformly tipped up, so it doesn't have any peaks that cause artificial sibilance or any kind of midrange nastiness. Its midrange isn't full or lush but very true to life and accurate in tone. The bass is punchy and tight, but drops off like a stone below 40hz. The SR-404's bass is a bit more bloated but is also substantially deeper. The SR-404 does have a significant edge in speed, being electrostatic, and its upper treble has a very pleasant airy velvety feel, even if its lower treble is a bit peaky and harsh. The K1k doesn't feel quite as refined in the highs, but it's also not quite as harsh and peaky either.
The O2 is a completely different animal. It has a much smaller soundstage than either the K1k or the SR-404, but its imaging is even more precise than the K1k and miles ahead of the SR-404. It has a full, rich, lush sound that is nevertheless extremely detailed and very fast. It's quite impactful for a 'stat when driven right, much more so than the SR-404 but still not quite up to the K1k. Its dynamic range, however, is the best I've ever heard, bar none. The O2 almost sounds supernatural and magical - the fullness and lushness sounds almost like it's a coloration - but it somehow doesn't get in the way of its tonal accuracy, hence the "almost" denominator.
I much prefer the O2 over the K1k or the SR-404. I think the SR-404 could be a good headphone if matched with a source that has the reverse coloration in the midrange, effectively neutralizing it - but failing that, it's not that great of a headphone in my experience. It's just too flawed, and whatever it does well (and it does a lot of things well, make no mistake) isn't enough to offset its one glaring problem. The K1k needs to be system-matched properly but its combination of raw detail, tone, soundstage size, and impact, make it a very serious contender with nearly unlimited top-end potential. The O2 though, in the right rig, just has a certain magic to my ear that the others lack. Every time I've listened to any other headphone I've always picked out things that were wrong and that irritated me, but listening to the O2 is like coming home. It just feels right.
