Most subs have a crossover built in, with a high pass output jack. You feed full mix to the sub, it sucks out the lows, then you feed that to your top cabs...

Alternatively, if you just need a little extra depth, and aren't trying to get the system as loud as it can go, you just feed the mix (in mono preferably) to the subs, and full mix (in stereo) to the top cabs. As long as there isn't a big forward or backwards difference between the subs and tops (phase issues), it works fine.

Yamaha mixers, and some others too, I think, have a dedicated mono out. Some of them even have a crossover for the mono out if your subs DON'T have an x-over...