I like the Uha-4, but it's not for me. I love the digital volume control. Battery life is excellent, no complaint's with the using it with a laptop using footbar/winamp etc. Crossfeed I have mixed feelings about, only impression I get is it "dulls" the sound, no matter what I listen too. As mentioned it is more of a neutral sounding amp, no hiss, no distortion at high volume levels, if that is the kind of amp you like for $179 you're getting a great deal.
In some ways I think my PK2's do not take full advantage of the amps capability, I am not a IEM can person, but I did try my copper pro turbines and if they would only stay in my ear lol, I probably be happy. But I like earbuds, maybe the PK1s or OK1s might work better.
The only things I did have some disappointment with and maybe it is because of the PK2s that there was no big increase in base or treble.
At high volume, best way for me to describe it, at volume very natural sounding with no distortion, I am starting to get the impression I am more of a person that likes color in my sound, with an increase in bass and treble and wide sound stage, or at least have the option to be able to switch it on and off.
Because of the reasons mentioned above, I will be returning the amp. If I keep it, it would only sit on the desk and never be used. I am glad I gave it a try, it's just not the amp that suit's how I would like to listen to my music.