http://www.seeko.co.kr/zboard4/zboard.php?id=cool_review&no=355 there are one reviews which includes some objective test result. and you will find the difference between X3 and some other DAP is not as big as the price.
Was just looking at the X3 listed specs on that page and they are showing 8 Vp-p output. For some reason I thought I had seen that listed as 4 Vp-p somewhere else. 8 Vp-p should mean that even higher-impedance headphones like the Sennheiser HD650s should be pretty happy with the X3's power output?