Subwoofers make all big speakers obsolete?

[EJ3]

Just started my fb2k foo_crossover journey integrating subwoofer to the KH310 mains.

What are the considerations/approaches to selecting the crossover frequency?

Secondly, what are the considerations/approaches to time aligning the subwoofer with mains? Is this step even necessary at all?

Appreciate any comments/suggestions. Thank you.

I can only answer question 2 from experience: if the sub(s) are downward firing and your main speakers are on top of them is likely the only time that I would say time alignment may not be necessary.