easiest way to hook up a sub is direct out into the sub and then from the sub out to the monitors.  works with the S10 as well.  
 
I also think trial and error is the right way but it involves doing mix downs and listening in a lot of different environments.  My car is my favorite place to check Bass.  i know what a fished radio mix should sound like there.  It is much easier for me to know if something translates once I have listened to it in my car and on my laptop speakers.  If it sounds balanced in both places I feel pretty confident.  I have 3 pair of monitor speakers in the studio as well.  vxt8's rokit5's and maudio cheapies.   
 
I personally have it set up this way.
 
Sound card ---> Switchwitch -----> 4 different outputs (1-VXT's, 2-Rokit's, 3-S10 Sub, 4-MAudio AV40's)
 
I can turn them on and off at will to compare a mix on each set of monitors with or without the sub.
 
I balanced by Trial and Error but you could run white noise through the system and then analyze what comes out.   Fill in the base until you have even representation across the spectrum.
 
Switch Witch
http://www.sweetwater.com/store/detail/SwitchWitch/

aglewis723
Thank you for the help everyone.   If I use seperate output ports on my audio interface (1 for monitors and 1 for sub), I will have to make sure the levels are exactly matching on both outputs, right?

That's why I was asking if there was a way to hook them up in 1 chain.... something like AUDIO INTERFACE > SUB > MONITORS or something like that.

That's tricky to set up, and to maintain the same relative volumes when you turn the system up or down. Most subwoofers alleviate the need to do this by supplying their own crossovers. You'll move your existing speaker cables over to the sub, and then you'll need two new cables to run from the sub's outputs back to the main speakers. The only adjustment you'll have to make is to set the sub's crossover frequency to around 60-70Hz and level, both of which will require some trial and error. 
 
The most important decision is figuring out where to put the sub. But that's a topic to tackle later...

Thank you everyone!

All my speakers have volume controls on them including the sub. I set the monitors to the 50% point and the sub is "to taste" ......balanced according to my ears.  Like Goldilocks.... not too hot and not too cold, just right.
 
The way it's wired.... as I adjust the volume in the DAW with faders, everything moves together in the world of speaker volume. Monitors get louder and sub gets louder..... and vice versa.
 
The sub has a crossover that took a bit of playing with to get it's adjustment right.....again, based on what I perceive to be right.

Came across this rather comprehensive how-to on subwoofer setup:
 
http://www.soundonsound.com/sos/jul02/articles/subwoofers.asp
 
It's from 2002, but the technology in this area probably hasn't changed a bit in the last decade.