@musicheaven: thanks for clearing that up. I did a quick read through the posted links about the ES9018xxx and the antialiasing filters. Seems, that it will be labor-intensive to tweak them for hardware manufacturers like iBasso and the likes. The only thing which gives me hope in that regard is what the user "twest820" @diyaudio.com stated:
The default fast rolloff filter is much the same as the linear phase "brickwalls" found on other DACs. ESS's slow rolloff has a noticeably wider transition band than the slow roll linear phase filters chosen by other manufacturers and hence is a good choice if one is attempting to minimize the amount of preringing the antialiasing filter creates in the output signal.
Intermediate phase filters with reduced preringing scored better, though not as well as minimum phase filters. Since ESS's built in filters are all linear phase this means the only compete option ESS has against Cirrus and Wolfson in this regard is for one to synthesize one's own minimum phase-ish filter and program it into an ES901x series DAC.
My conclusion of the above statements would be, that there is room for improvement because it still seems that it's possible to implement different filters!? If I'm wrong, please excuse me, I'm not a sound engineer^^