Today, I saw a great article on Prosoundweb about the technique of using aux-fed subwoofers in your sound system setup. This is where you assign a post-fade aux send output to your subwoofers as an independant mix. So, for things like kick drum, bass guitar only those signals would end up at the subwoofers while other signals like vocals, piano do not go to the sub mix. The result is a cleaner mix.
Here is the link to the article:
Provided with the article is a great diagram that shows the hookup schematics for a Traditional Subwoofer system and an Aux-Fed Subwoofer system. Personally, I have tried the aux-fed subwooder technique and it works great. (see diagram below). Happy Mixing!!!