They are for accepting STEREO signal at INPUT.
Example:
Row 1 amp panned L
Row 2 amp panned R
Mixer— > if you use mono Cab…your result Is mono signal…
I can’t remember who it was that posted this in Discord- I think it was Max from NDSP:
" So in the Mono Cabs, the Stereo input signal gets fed so that L goes into the Left speaker and R goes into the right speaker. When the Pans are set to Center, the signal is summed to Mono on the output; when the pans are hard left and hard right, the signal goes as Stereo on the output.
On the Stereo Cabs, both IRs (Speakers) are present on the L and R channels meaning the microphone blending will apply equally to both L and R channels. When the Balance is at Center, both speakers will respect the full stereo image, allowing you to have the same microphone blending and tweaks duplicated on both channels at the same time.
The balance fader will allow you to pan each of the Stereo IRs independently, following Stereo output pan laws.
A good example on how to use Stereo Cabs is with a Dual Amp setup, used in Rows 1 and 2 respectively, with a mix point to Row 1 and a Cab receiving that signal which comes hard panned. With the Stereo Cab you can use only one microphone (turn off right speaker/mic) and said microphone will affect both L and R channels (both amps) equally, without having to add a separate cab and tweak again. If you use both microphones, you will be running 4 IRs in total, 2 on the left and 2 on the right, controlled by the same UI. It is basically the same that we do in our plugins with the Stereo In Switch."