Assuming that the headunit and the poweramp crossover slope is the same (12 db/octave), is there an advantage to using either one for the crossover point? Is one setup more efficient than the other?

Let's say I want a high-pass frequency of 75 Hz, I set the headunit crossover point at 75, does it matter if the poweramp is set for full range or maybe at a crossover point of 50 Hz? Do I gain anything by not having the poweramp "play" full range even though the low frequency signals aren't reaching the amp?

 

In general, the amp will have cleaner crossovers. Try to do all your settings at the amp. But if they have the same specs, there shouldn't be a difference.