I have a K701, have used Q701, and just auditioned a K545 for a couple of weeks.
Lukalop, if you are only going to use the headphones at home AND have a source (CD player, stereo system, etc.) with a decently powerful headphone amp section, get the Q701. The soundstage is much wider than the K545. Also, there is more depth -- some instruments are clearly further away than others -- whereas with the K545, all the instruments are about the same distance away. Instrument separation is also much better with the K/Q701. You can sense spaces between each instrument. With the K545, the whole band comes to you more as a solid mass. I always thought the Q/K701's sound as "shower," where you can feel each drop of water separately. With the K545, it's more like a stream form a hose.
But if you are going to use the headphones on the go, definitely stay away from the Q701. Also, if you are using your iPod / iPad / cell phone / computer as your music player, Q701 will not sound good AT ALL. K701 from my cell phone or computer sounds as if I'm standing at 1 end of a tunnel, and there is a tiny boombox at the other end. Tinny with an inordinate amount of reverb. At the very least you need a decent CD player or a stereo system with a fairly powerful internal headphone amp to drive the Q701.
K545 is very clear, almost as clear as the Q/K701. It has more bass than the Q701, and quite a bit more than the K701. For rock and harder fusion (Tribal Tech, the Aristocrats, etc.), K545 is more enjoyable (at least for me) than the Q/K701.
I noticed that all 3 require quite a bit of burn-in. If you do not think a headphone's sound changes as they are being used more and more, you can ignore what I'm about to say … but until about 20 hours in, the K545's bass was a little anemic. I fidgeted with cup angle and placement to get acceptable bass, but on some songs the low frequencies were almost inaudible. After 20 hours or so, the bass bloomed. At that point, I noticed this: I get to work, I put the K545 on, and it takes about a song for the sound to settle. Kind of like a mini-burn-in. If I used the headphones continuously, at the end of the workday the bass would be almost overwhelming. I would adjust the cup angle and placement to *lower* the bass quantity. This makes me think the K545 takes much more than 20-40 hours before the sound settles. After 20 or so hours, the volume increased, too. I think certain components get looser and create more volume and low end. Coming from guitar amp experience, where speaker burn-in is a recognized phenomenon, I would say the glue that attaches the cone to the chassis is probably getting softer.
Also, the cup angle and placement, not to mention the seal, greatly affect the sound. The great thing is, the cups swivel up/down and also forward/backward, and they stay put once you find a spot.
All my listening at work was done straight from my phone (HTC EVO 4G LTE), and at home I compared the K701 and the K545 straight from the headphone out of my Sony CD Player. With a dedicated DAC/amp, the result would most likely be different.
On a different note: I ordered a Sony MDR-7520. Would be interesting to see how it compares to the K545.