Iodine, I find myself in the same position as you, I have k701s and I was also thinking about buying another headphone to have as a second reference. I think the Beyerdynamics look more comfortable and more like the K701 style than the Ultrasones, but also the frequency response is very similar to the AKGs at least according to the frequency graphs at headphone.com. Anyways, no matter your choice, if you really think there is too much bass you can always put on some equalization. I use foobar2000 with the winamp dsp bridge and use the Electri-q posihfopit free parametric equalizer through that. Lots of people talk about each headphone's frequency response signature like you're committed to it but you're not, I think you are more committed to the stereo imaging, fit, and appearance, so just don't worry about it I'm sure you'll make a good choice. It might be worth it to just get something like a refurbished HD 280 for a closed phone so you can buy some other open phones haha. I personally don't know whether to get HD 555, HD 558, or a KRK 10S subwoofer.