I have the Shure SE530, and what they need most is a treble boost. Badly need it. I would not consider any amp wtihout that feature. Even with it, my Shure are way below my Phonak PFE 232 in detail and enjoyment. But I can understand some like the ultra warm smooth midrange of the Shure.
Also, the Arrow gain settings I use differ depending on the earphones. There is an interaction between the two, such that the sound signature of gain 0, I, and II differ as you change headphones. On my HD650, I want the extended and smooth setting of gain I (IIRC) but for the Phonak, I require fuller midrange, so I use the 0 gain.