With the Xonar Essence One ASUS have definitely taken the fight to the DAC and headphone amplifier manufacturers. The design provides excellent sonic characteristics and qualities and furthermore it is easily customizable. The headphone amplifier section of the Essence One is definitely one of the better out there both in terms of power but also definition, unfortunately it is paired with an annoying volume control that does not give the necessary control. Looking into the DAC section of the amplifier reveals that ASUS means business with the One.
The 192 kHz / 24-bit DAC section easily bests the NOS 1704 UK setup in the HiFiMAN HM-801 and is ahead of the AKM4480 equipped Fostex HP-P1. The question on everyone's lips is of course whether or not the Essence One is worth the price difference compared to the Essence ST/STX sound cards and the simple answer is yes, for anyone with a set of high-quality headphones or studio monitors. The Essence One punches above its weight, and if you can live with some of the quirks like the somewhat annoying volume control, odd shape, and lots of blue LEDs then Essence One is brilliant value.

Upsampling with the Essence One is a hit and miss, it simply does not improve anything, but it is a borderline who-cares situation since it sounds so brilliant with standard CD quality source material.