I'm trying to set up my HifiBerry AMP+ on a Raspberry Pi 3 Model B+ running NixOS and the raspberry pi kernel 4.14.70. After I added dtoverlay=hifiberry-amp lines to /boot/config.txt, absolutely nothing changes. The AMP doesn't turn up in aplay, the snd_soc_hifiberry_amp module isn't loaded automatically (I can load it manually, but it doesn't do anything). dtdebug=1 doesn't result in any dmesg output either.
How can I debug the problem?