There is a reason why that switch is there on sub-woofers. Off topic a little, if you have a cell phone and you are looking for a bluetooth headset you will run into a process called "noise cancellation". What is that and what does that have tot do with subs? The answer is everything.

When 2 sources of sound send audio into a room hitting the same space at the same time it causes cancellation. That is the principle behind the noise cancelation devices. This is not a problem for your main speakers if your system is running stereo. But it is a big problem for subs. Usually subs are wired to run mono, and the wavelengths of the sub frequencies are huge. For example a 20Hz signal has a wavelength of 56.2664 ft. How big is your platform? is it 40' or 60' across? A  30Hz signal has a wavelength of 37.5109 ft. So do the subs placed 40' apart cause cancellation? Yes big time. The solution change the polarity on one of the subs. That way when the one sub is on the outward movement the other sub is on the inward movement. You will notice a big change in the sub frequencies.