I have galaxy s4 i9505 and galaxy s1
after poweramp i use Neutron
and i really like, after power amp i felt improvement in sound
I really like Shuttle, it has a beautiful and clean interface. I can't comment on the EQ quality because I keep it flat and I also can't tell the difference in sound quality between Neutron, Poweramp, Winamp, Samsung, and Jetaudio.
there's quite a bit about it in the android-phones-and-usb-dacs thread