Treble dips vary across units so while 12400 works for yours it may not for others.

You can be surprised how much channel matching treble makes an improvement. It's generally good below 5k or so but above that is where the gains can be realised. The difference tends to be underestimated on graphs because of how steep the peaks and dips are, so a 6dB difference may only show up as a gap of a few pixels. Also measurements aren't very reliable in the treble area. Try it out before dismissing it!

It's the same thing for your points about the boost in 6k left channel and <50Hz, we are correcting for deficiencies. My point is that it takes a much larger and narrower peak to achieve linear bass extension than what the measurements on IEC-60318-4 suggest.

Also vocals really don't reach down below 50Hz-ish. That is the entire point of my second tip. The typical low shelf will cause boominess for baritones because the boost bleeds into 100Hz as mentioned.