Honestly, BH makes my Colorfly CK4 , C3, and Hifiman HM-601's headphone outs sound average in comparison, those are not cheap players. Every source I've tried gains some improvements, even the mighty Clip+, ipod touch to my old school Sony Mini Disc player. The amp takes what's there and improves the signal for you, expands the stage, increases detail, clarity, all it needs is a decent signal to extract from, how good that original signal is will depend on your outcome. However, of course what ever is being driven will sound better than the headphone out alone, whether it needs to be amped or not imo. Many members reporting positive results with low impedance IEM for example. You can see this by all the positive reports members are giving confirming.
I do highly recommend using line outs though, only headphone out if LOD is not available.