Confused: M/S in Phones vs Main control room output


In my project, I have a mono bass guitar (EZbass). In the Control room I have phones output (for my headphones) and Main for my studio monitors.

I inserted the bx_solo plugin on the Phones and Main output to listen to the Side channel only.

As expected, on my studio monitors I hear no audio (since the bass on on the Mid channel)

However, unexpectedly (?), when I audition the Side channel on the Phones output then I still hear the bass. How can this be?

ps. The stereo-width knob on the bx plugin is set to 100% meaning: no change.


I can see much more plug-ins on the Main. What happens, if you bypass them all and keep just the bx_solo?