The theory as I understand it is to allow "room" for the crossover slopes to blend together rather than assume a straight-line cutoff at the crossover point. On the other hand, my Studio/60's are 3dB down at around 37Hz or 39Hz, and I have them crossed over at 40Hz because it sounds best to my ear (possibly due to relying solely on them for a couple years before ever getting a sub -- but, particularly for music, I preferred the 40Hz point to higher crossovers). That's why I suggest experimenting a bit. For the center, I'd obviously not go below 80Hz (since 60Hz would create a hole). You might see if the center blends with the sub better at 100Hz or 120Hz, just for kicks.

------------------
gonk -- Saloon Links | Pre/Pro Comparison Chart | 950 Review