Hi
@nazure! Welcome to ASR.
If your FiiO Q1 can achieve your desired volume levels with the HE400se and there's no obvious distortion or clipping, then it is sufficient for these headphones.
That's really all there is to it.
If the FiiO Q1 plays clean but not quite loud enough, then check that your computer is sending the full digital signal amplitude (0dBFS Peak) to the DAC.
Often times, there's a volume slider not maxed out somewhere or the tracks are mastered with excessive digital headroom, which can lead to insufficient loudness.