Good question - depends a bit on placement, and whether you want the sub to play all the time, or just on movie effects, and/or whether you want the whole system to play louder. Also, you would need to be looking at a pretty serious sub to really augment a pair of Tritons in any way. Have you tried just turning up the bass level control on the Tritons a bit?
When adding a sub, if the sub is gong in the corner, side or back of the room, I would suggest a LFE-only approach: Set sub to "Yes" and keep everything else the same. This will direct all channels' bass to the "Large" Tritons but send LFE channel information to the sub.
If you can place the sub toward the center-front of your theater you can get the sub to play more and increase the system's overall playing ceiling. Change the L/R Main Tritons to "Small" with a 80Hz crossover setting and subwoofer to "Yes".
