I've tried different setups for crossovers. Ascend 340s (rated down to 55hz) fronts, and Ascend 200s rears.
The latest setting was 60hz (using HK635). My center channel is sitting on a shelf in a cabinet, and I think there's some cabinet buzz/rattle at lower frequencies. I have an Auralex MoPAD on order to isolate the center speaker from the cabinet.
I imagine the base localizes a little better at 60Hz than at 100hz. I watched "House of Flying Daggers" last night, turned up loud, and when the horses are galloping across the room, the bass was tracking the sound pretty well!