>>I was concerned enough about this problem that I asked Ian and Tom to run sweeps on a 500 and 600 combined in the anechoic chamber 2 weeks ago. Those 2 subs are close enough in design that phase was not a problem and as I had hoped their combined response was additive with no cancellations.

Cool !!

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