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.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.