My current setup:

ALSA dmix (period_size 480, periods 3, rate 48000) --> spdif out --> spdif in --> zita-a2j -p 96 -n 2 -p 48000
--> jackd -d netone -p 192 -n 2 or zita-j2a -p 96 -n 2 -r 48000

480*3 is definitely not a multiple of 96*2 although it seems to sound fine.
Should it be a multiple of zita-a2j buffer size?