There is no stereo with subs. You can hook em up that way. You can position and pretend they are stereo. But it is all mono.Sorry not correct I have two subs, running stereo, each on the outside of my mains, they run individually off the line level left and right channels. You can clearly hear stereo subs when the recording dictates for it to be heard (especially orchestral works, tympani in back right channel double bass far right). CD has over 100dB of channel separation all the way down to 0hz.
https://cnx.org/resources/e9e985c5b348f2d44cfbc3947900dc96475ba08c/graphics1.png
Different story with vinyl, as it has at best only 30db chanel separation at 1khz, and almost no!! channel separation at 20hz (in other words mono)
Cheers George