Byfo, very few recordings are stereo below 100 Hz, but the final answer in my opinion is related to the low-pass filter frequency and slope. (The low-pass filter is what rolls off the top end of the sub's output, and usually the frequency is adjustable but the slope is fixed.)
In general if the subs are audible as distinct sound sources (while the mains are also playing), then they're putting out significant energy high enough into the lower midrange that they should be positioned fairly symmetrically and near the main speakers. If you can't detect the subs as separate sound sources, that means they're crossed over low enough and steep enough that they can be positioned with priority given to in-room bass smoothness - and they're probably operating below the region where stereo is a factor.
As a ballpark rule of thumb, if the subs are rolling off at 24 dB per octave around 80 Hz or lower, I wouldn't worry about positioning them with stereo in mind. But you can't necessarily trust the frequency knob on your plate amp, so if your ears tell you the subs are audible reposition them accordingly.
In general if the subs are audible as distinct sound sources (while the mains are also playing), then they're putting out significant energy high enough into the lower midrange that they should be positioned fairly symmetrically and near the main speakers. If you can't detect the subs as separate sound sources, that means they're crossed over low enough and steep enough that they can be positioned with priority given to in-room bass smoothness - and they're probably operating below the region where stereo is a factor.
As a ballpark rule of thumb, if the subs are rolling off at 24 dB per octave around 80 Hz or lower, I wouldn't worry about positioning them with stereo in mind. But you can't necessarily trust the frequency knob on your plate amp, so if your ears tell you the subs are audible reposition them accordingly.