Before trying anything else and assuming that the IEMs go 1/8" to 1/4" via adapter (3.5mm up to 6.35mm), I would try a different adapter. Even if it works fine in everything else, it sounds to me like there’s a problem with your adapter rather than the digitakt but I’m just guessing based on the evidence you provided.
also:
I don’t understand this, is this an unrelated thought? Slicing is already available in the most recent official update which has been out for a month or two now, so no workarounds required, just visit the product support page on elektron.se and make sure your firmware is current.
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Good luck.