Why do you think that? I have the preamp set below the total gain output. The clipping is the sound of the drivers over extending (you know the popping sound when you push them hard) and not the typical amp distortion people are familiar with. I know the difference. I am headphone newb but I've been fooling around with car/home/club audio systems since I was a kid.
Throw up the low freqs very high on an EQ and lower the preamp. Then turn up the volume and you will get your drivers popping too. The Ultrasone drivers are too tight. I'm doing the exact same setup on these JVC and they are handling it like a champ. My old ATH-AD700s handled it like a champ too.
I drove the low end extremely hard when I had them. Maximum Mach3bass and BBE setting on my Cowon S9 at the time. Never did I hear this popping you're referring to. Either the headphones are damaged or your amplifier is clipping due to compensating for the changes in your EQ.