Thanks for your DIY a few questions, please.
I just want to clarify your wiring and installation of the drivers of your subwoofer by showing a diagram attached.
this image shows only 2 drivers, they other 2 drivers are identically mounted behind these 2 speakers.
Questions please refer to image:
1. these 2 drivers are wired out of phase in parallel.
2. the other 2 drivers also wired out of phase in parallel.
3. (1) and (2) are then wired in series.
if your installation is different, pls supply an actual wiring diagram/image of your DIY