If ARC measures distortion, it could be your subs are more linear than the 180 below 80Hz so ARC hands those duties to them.
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M2 coming
A-LFR/1000-8/500v4x2
1000-2,3,1500-3
100,160,QS10x2,800,M5x2,M3,M50 v4
had v2
N2,N3 maybe