I set my amp to do it and its 100hz crossover, 80hz does not suit my speakers. And have the phase set to 0 I think, but thats your preference. I think most will say let the amp do it.
Hi, just to note on my sub there is a variable or bypass switch which I have this set to bypass which lets the amp sort out all the necessary settings.
It set my sub to -15 which is the lowest, and had to turn it back to -6 and it set my fronts and rear to large when the are bookshelfs, not capable of giving out real base, so had to turn those back.