I got the O2 (Mk1) after the K1k, and for the time being it put a stop to my headphone purchases. Right now I'm trying to max out the rest of the signal path within budget limitations, and the O2 is so brutally revealing that it lets you know right away what is wrong with your equipment. The O2 is also rather different in its presentation from the K1k and IMO less colored - more extended on the bottom, far more liquid and fluid, and less steely up top, but also less vibrant and less dynamic. It's up to your equipment to make it vibrant or dynamic, and up to your equipment to basically voice it whichever way you want. Which really means that your equipment has better be good. The K1k thrives on lush, liquid tube gear, but the O2's sound can vary from bright, dark, steely, wooly, liquid, dry, thin, think, basically anything that you can think of, all based on what it's mated to.