PSW111 crossover

[Doc99]

I bought an PSW111 that I was going to use for a 2 channel system, and wanted to set my subs low pass to around 60hz. I noticed now that this model lowest setting is 80hz. The sub is rated at 38Hz-250Hz. I don't understand why the low pass dial would not give you the option of going lower, like 70, 60, 50, 40 hz. Just wondering before I return this. It makes no sense to me??