This one was the benefit of EQing by
wait for it
actual notes/keys.
Seems so stupidly obvious now. Kind of realised by accident in a recent mix - yeah, instruments have natural ranges that they resonate in and occupy in the mix which one can emphasise or de-emphasise to give them their own space while being mutally supportive, but there are bloody notes as well! If things are murky/phasey/gay sounding them maybe its because the frequency ranges increased or decreased arent working with the frequencies of the fundamentals of the bloody keys being used!??! Turns out it works (in a you-should-track-it-right-to-begin-with sort of a way and so never have to do this, but it helped out with this project and I can see where not doing it probably caused problems in previous ones).