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.
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M60ti, VP180, QS8, M2ti, EP500, PC+ 20-39
M5HP, M40
LFR1100 actives picked up Friday 13th