as a follow up, i got my gakken today! funny little device. here you can find a webapp (chrome only) to upload a more feature full firmware config, that lets you disable the reverb for example:
https://webgk.gakken.jp/otonanokagaku/nsx39/appli/00/
on my device the headphone is quite clean (apart from a bit of white noise) there is no hum or anything. maybe a tiny weeny bit of digital noise, but really ok. disabling the reverb makes it a lot cleaner too, since that added some artefacts when decaying...