I've been thinking about the added M-165's below, and I think what will be ideal is to power them with an separate and inexpensive plate amp. That way the lower end can be dialed in and balanced phase wise, and output wise with flexibility on the crossover point.
This also keeps a large cap value from being inline with the M165NQ mid-bass driver. This can be more effectively done with a small cap value prior to the amplifier and at a lower cost.
And the high quality passive parts needed for the low (100Hz range) crossover point will be more costly than a good plate amp and not have the flexibility that a separate plate amp can provide.