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Right after I posted those 2 videos, I noticed the D note is still a bit less powerful than the rest (noticeable in the scale video). I had a thought that what if the bridge is just really slightly off, even though everything looks fine.. It made full contact.. was where it was supposed to be in relation to the bass bar and sound post as well as the notches.. string length was good. When the instrument arrived it already lost its tuning and the bridge was in a totally random / incorrect position so I just eyballed it in then and never moved it since..
But I never actually measured it against the fingerboard.. (you know... projecting the sides of the fingerboard to the bridge) and I had a feeling the luthier who made the instrument intended it to be there as a continuation of the fingerboard projection or even slightly to the left of that... So I put a ruler on each side of the fingerboard and noticed that even though it looks to be centered.. it is like 2 millimeters to the right (towards the E string)... Losened the strings.. moved it that much to the left... did nothing else.. tuned it up again... and now my ears hurt :))))) Because everything became louder and ringing.. and pretty beautiful... even that D note.. so I think I may have solved the problem!....