I much prefer to take six db off one or both if a pair off a mono amp... I use two front subs that are timed as one, the phase is nigh on identical to within +/-3 degrees at the listening position, so to match them to the mids I knock em down and adjust both channels delay as one and to get the crossover roughly where the phase is good for all three drivers (basically I lump the two subs in as one because their phase responses are so similar in my cabin at the listening position