I believe that Cambridge only rates the S50 down to 80hz, so probably best not to go lower than that. The S30s are rated down to 55hz and so could probably work OK with a 60hz crossover. Basically, set it lower if it sounds better to you. A lot of people go for 80hz because (a) that passes off much of the low frequency work from the receiver to the sub amplifier (takes a little load off the receiver) and (b) no higher than 80hz because the sounds coming from the sub are more likely to become localizable. Still, whatever sounds best to you.
But don't to below the crossover settings that your receiver/Audyssey came up with initially. Below that, Audyssey won't set any filters for your speakers to improve the response.