My Soundcraft mixer has all the circuitry on one big PCB. It also has an external power supply. Make what you will of that...
Even a toroidal transformer is trouble when you have sensitive mic inputs close to it. OTOH the best preamp in the world manages to have the PSU on the board - but that's only two channels and I'll bet the sensitive end is as far away as possible from the transformer. Also the transformer is potted and clearly overall screened.