IMO DC elite have a little U shaped tuning compared to other dac. I keep finding Dc elite add treble and bass which is fun sounding overall. Unfortunately if your iem already bright this can increase its brightness. I just tried the D2 , yeah it wasnt much difference from the original. FC6 is very rich sounding and thick from low to top which is quite addicting and enjoyable for long session. But the stage and holographic ability is definitely less than DC elite. I think DC elite is still the more resolving one , i have not met other dongle as resolving as DC elite for now.
OK, perhaps your digital source is adding jitter noise or some other digital artefacts? Its a well publicised fact that noisy digital = brighter sound.
My ie900 sounds great with the DC Elite, not sure if its classed as a bright IEM though? Here is a frequency response graph I found. It does look a bit "spiky" in the 6-12kHz region.