KitGuru tried the ASUS Xonar U7 Echelon Edition
A quote from the article:
Last year ASUS enjoyed both public and critical success with their Xonar U7 USB sound card. For the enthusiast user demanding higher grade sound quality, adding an external DAC can be a good investment. Driving an expensive audiophile grade pair of headphones from the basic sound chip on a motherboard can also lead to less than optimal results. Asus have addressed this as the Xonar U7 incorporates a dedicated headphone amplifier. Priced at only £80 (http://www.amazon.co.uk/ASUS-90YB00BB-M0UC00-XONAR-U7-ECHELON/dp/B00HASV12Q) , it won't break the bank either.ASUS Xonar U7 Echelon Edition Review @ Kitguru