While I am not someone who gives a lot about dacs and amps and how they differ in sound (barely noticed a difference between a solid state amp to a MT-602 hybrid and MT-602 to a TA-26 OTL) I find a 4 way amp for 30 bucks (less than 10 bucks per “amp”) to review sensitivity IEM’s 10 times its price a little strange. Especially when I look up the HA400 and see it has an output impedance of 80 Ohm which might influence the bass response of some dynamic drivers.
On the other side you are doing reviews for such a long time that you would notice if the 4 way amp changes sound too much.
Edit:
Because we all like squiggly lines so much. Seems like the HA400 adds a touch of warmth and reduces the treble.
https://reference-audio-analyzer.pro/en/report/amp/behringer-ha-400.php#gsc.tab=0