You can use these sockets as mix sends to your sub woofer. Take a stereo jack plug, open it up, solder a link between tip and ring, (to enable the internal amp to continue to operate as before) and then connect a screened cable to the sleeve (screen) and the inner core to the two linked terminals tip+ring. On the other end, attach a suitable plug for your sub woofer and plug it in, then the master fader on the Studiomaster will fade the subwoofer up and down in sync with the main built-in amp.
I'd prefer to do it this way because using the monitor bus means the master fader hasn't got ult