The SPDIF is sourced after the digital domain master volume control. There is the analog master attenuator setting on the back of the mixer, but it only adjusts in crude increments. There's no way to source the SPDIF from the REC feed, for instance.
The master out volume knob, by the way, is unity at like 2:30ish o'clock if you're running your channel faders' unity as max. I believe Pioneer originally meant people to run the faders as unity at 7 out of 10 with the master volume unity at max, but most don't do that.
Generally, I think it's a bad idea to give boost over unity on the master volume, or to have the guest DJs access to boosting the system volume. I'd just suggest you stick the master at its respective unity to however you use the channel faders and control your sound system elsewhere.