FabFilter

Hi all.

Whilst mixing today I've come across something unintended and I'm struggling to understand why it's happening.

I have an instance of ProQ on a mono bass signal I've just recorded (not a mono track as Ableton doesn't have this option)
I have added 4/5 narrow band cuts and a low pass filter (to get rid of annoying top end).

Whenever I'm mixing I have the BX_solo plugin on my master to listen to the mids/side signal. When soloing the S on the master I could hear the bass audio. I checked my inserts and there's no plugin on the bass track for stereo imaging tricks.

When flicking the output of ProQ from L/R to M/S that seems to prevent the bass signal going to the sides as I can no longer hear it.

I'm running Ableton and as we all know Ableton doesn't have mono channels, only stereo so I'm wondering could that be why the bass signal is sent to the sides? if so could someone explain why?

I feel like this king of things can make or break a mix and I'm just scratching the surface. It also excites me to understand why such things happen and hopefully be able to use these to my advantage.

Many thanks