The amp I was using is just the headphone jack of my Pioneer home theater AVRs. As I have tried amps/DACs at home, I have rolled through many amps over the past few years (all solid state) and I have found the differences to be very subtle. Maybe its just me, but I find it very difficult to detect differences and so I have never been blown away before.
No, the Asus didnt blow me away either. It snuck up on me. At first all I could detect was a slightly leaner more controlled bass response. Then as I listened more, I began to notice the Asus sounded more dynamic - if the Pioneer and Asus were both identical polo shirts, the Pioneer has been through the wash 100 times while the Asus is new, if that makes sense.
The Asus sounds more vivid, snappier, faster, cleaner where the Pioneer sounds a little flat and lifeless. It's a very subtle thing though - I really have to focus to detect it. But then when I turn off the Pioneer and just kick back with the Asus, it just moves me more. The Asus is more musical. I don't mean to saythe Pioneer sounds bad - it's hard to describe subtle differences without hyperbole because it's the only way to express the differences. But the differences are so small as to be near,y imperceptible without extensive listening and focused concentration.
It's a small thing but for me it's worth it. Not to be cliche, but the Asus just has that "toe-tapping" quality that my Pioneers lack. Amd it has it without sacrificing detail or adding coloration/distortion. It just sounds more natural to me - which is kind of hard for me to admit because I am quite an objectivist.
EDIT: Other Headamps/DACs I have owned and/or tried in my home include - Peachtree Nova, Schiit Asgard, Lyr, Objective2, Violectric V90, V200, V800, Burson HA-160D, Woo WA6, WA7, Benchmark DAC1, Grace M901, Cambridge DacMagic.
I have detected differences in frequency response before, but never do I remember noticing the more subjective differences I tried to describe above regarding the Essence One Muses.