I run the input to the soundcard off the subs. That way they are assignable with a switch. Assigne which channel you want to record via the sub, use the sub 1/2 to control the record level, use sub 3/4 as the feed to the mains bus to control the monitor level independantly of the record level.
To monitor the output of recorded tracks, bring the ouput of the soundcard to a couple of channel strips, and assign from there to the mains.
If you have a soundblaster type card, make sure to unchek the "what u hear" option so you don't set up any feedback loops and fry something.
Take some time with the block diagram of your mixer and explore the possibilities!