I wonder if the argument for using high level inputs is to have confidence that there isn't a delay between the low level signal going to the sub and the amp output going to mains etc... that can't be adjusted out with typical sub controls ?

Seems unlikely but not impossible... I see what appears to be interchangeable use of phase and delay when the two are actually only related at a specific frequency.

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8