Flats:
On a P, in a mix, yeah, they work.
Nothing else like them, plenty of great recordings to prove the value of that tone...
Flats on a J...no thanks.
I'll play flats, when someone requests it.
I even like and appreciate them for the right thing, when I hear them.
I keep one bass strung with them; either one of my old P's, or one of my old G&L L1000's.
I keep a set of both LaBella 760FL and DA Chromes on hand.
But, it ain't my thing, at all. It's a "does the project need it" thing.
Since it is about me, I'd never keep flats on something I play as regularly as my Sadowskys.
I don't have a Sadowsky P, and I sure wouldn't go down the road of rationalising one, just to keep flats on it