I know it doesn't exactly address you flat frequency response issue..... but I'd say just try different cabs and see what YOU like for YOUR circumstances.
I've tried 2x10's, 4x10's, 6x10's,1x15's, 2x15's, 1x12's, 2x12's and combinations of them..... and by various different well known manufacturers including BF.
My PERSONAL faves have ended up being the old Mesa/Boogie Diesel 2x15 and the more recent (& more portable) GenzBenz Nx212T. Why? Because they have most accurately produced the sound I like to hear when combined with my basses, amps and pedals of choice.
Ultimately, no amount of engineering/physics talk will change how YOU hear it.