@Michiel Kemper - I had a bit of trouble understanding your settings at the beginning of the clip but from here (laptop speakers) it sounds almost as it should with your phone mic off center like that. Curious if it would be different more straight on. You also mentioned you had the Master dimed? Wow.
I also sometimes do the Treble on '0' trick on my Dual. Like @endofall I have a R modded to C - no boost needed. I understand he sold his and kept his Triple. I have very little experience with the Multiwatts, especially at volume.
One thing to note, if you are not already, is that if you play with the FX engaged, patch the send/receive with a patch cable. That should help. That's how I run mine because I like having the master volume control. But I'm more like Gain at 3pm, Channel at 11am and Master at 10am.
Yours sounds a tad thin but may just be where your camera is.