ALSA loopback's clock is fed from audio signal of ALSA applications which cannot have a regular clock.

I haven't tried alsa_in on any hardware soundcard, yet.

I don't see actual xruns from alsa_in or alsa_out, but I hear pops and crackles.

Would the following setup cause pops and crackles?

1. Vivaldi web browser, mpv, mplayer, ... --> ALSA dmix --> onboard SPDIF Out --> SPDIF In of Creative Sound Blaster X-Fi HD USB --(alsa_in) --> netjack1 ==(ethernet)==> Jack ALSA backend --> HiFiBerry Amp2 --> Speakers

2. netjack1 --(alsa_out)--> headphone jack of X-Fi HD USB --> headphones

-----On Raspberry Pi 3B+

jackd -P 90 -d alsa -d HiFiBerryAmp2 -p 512 -n 2 -P -r 48000
jack_netsource -H 192.168.30.2 -o 0 -i 2 -O 0 -I 0 -n 2 -N guardian

-----On This Computer
jackd -P 90 -d netone -p 512 -n 2 -r 48000
alsa_out -j headphones -d headphones -p 512 -n 3 -q 2
alsa_in -j spdif-in -d spdif-in -p 512 -n 3 -q 2