Headphone output heavily distorting on C400

[Maris]

Hi all,
I have my Fast Track c 400 bought used fro some 4 years now. But I have barely used it because I bought it for possible remote recordings. Now after some time I put it on a thorough test. My setup: Windows 10 x64, C400 with 1.1.7 x64 drivers. Everything worked just fine, except one thing: the headphone output had an awful distortion when listening to any commercially released music. That is to everything where the peaks frequently reach 0 dBFs. At the same time both Line outputs sound briliantly. I connected the C400 to 4 different PC with Win 10, Win 7, installed the previous drivers 1.1.5, used different headphones (Sennh. HD 600, A-T ATH-M20) but the situation never changed. The single way to reduce the headphone distortion was gradually reducing the Software Return volume. But that of coarse reduces the Line Output as well what I don't like at all. And with acceptable distortion the headphones volume was rather low.
Has anyone expierenced headphone distortion when listening to quality full scale recordings (Pop music) with C400, C600, or other AI from Fast Track series? I guess that my C400 has got simply some electronic fault. It could not be a driver's fault, because then anyone would blame them for that?
I used different DAWs for this test as well: Nuendo 6, Sony Vegas 12, Sound Forge 11, VLC player, switched from Windows Classic drivers to ASIO drivers. Nothing helped
I'm sorry for my long post!

Maris