I use HM-901 with Balanced amp card for both my CIEMs and HE-6 (!). With sensitive IEMs you will of course have less and bigger steps in volume (since 901 sports a true stepped attenuator), but I can live with it.
To me, the point getting 901 is that it performs superbly as a stand alone portable unit as well as being able to deliver very high SQ (high end SQ in my opinion) from line out in a desktop scenario (feeding a HQ amp).
The line out SQ is simply jaw dropping good. When the rest of the chain is high end quality, you will hear what that little box can do. I literally dropped my jaw the first time I heard it through Burson Soloist to HE-6 with hi res music. This was even as a DAC (fed from my SqueezeBox Touch). SQ is even better when playing onboard files.
However, if an external portable amp is better than the Balanced amp card, I don't know.
I don't have teh Balanced Amp Card, though I feel my iBasso PB2 is very simmilar and on par with the Balanced out of my NFB10ES2, the only differance I can hear in the two is in the bass. With the NFb10ES2 being a touch tighter, though with my HE 4 it could be the Extra Head Rooma nd power that gives the Headphone a Tighter Bass out of the NFB10ES2
though for balanced amps go, my iBasso PB2 with LME 49990 OP amps is end game for me! The battery life is great, the sound is great and yes it will run my HE 4 to some very LOUD levels :3 I'm very happy with it
Though I don't know how it will compare directly to the Balanced Card within the player