Actually this is not a matter of taste or an overestimation, it's a pure fact that AKG K701/702/712 etc. are not so easy to drive. I own U7 and K701 so I know they don't pair so well, especially for high-dynamic music. My thoughts here: https://www.head-fi.org/threads/asus-xonar-u7-dac-amp-impressions-thread.669127/page-8#post-10114012. However, feel free to read Tyll's notes here: https://www.innerfidelity.com/images/AKGK701.pdf (90dB: 0.32V RMS=1.57mW; 100dB: 3.2V RMS=157mW) and ASUS notes here: https://www.asus.com/ro/Sound-Cards/Xonar_U7/specifications/ (1.3V RMS = 26.4mW). Usually people listens to an average RMS of 85dB which means having the loudest sounds from the songs at around 95-100dB; also, you always need a power reserve of 5dB or even10dB for songs having a better dynamic or recorded before loudness war. Cheers!