I can tell you that the solution that gets flat impedance is Butterworth. This is a constant power solution.

Putting 500Hz and 6 ohms into a decent 2nd order Butterworth calculator comes up with 37.5uF and 2.7mH for both arms:

2-Way Crossover Calculator / Designer
This ought to give you a flat impedance. You will take your beating on the bass Fs peak. And you might need to do a bit of Zobel impedance equalisation on driver inductance.

I shamelessly stole this idea from the

Rogers LS5/9 monitor. Passive LR bafflestep and otherwise a smaller bass coil.

Where the theory departs is in bass loading, here reflex, and not really suitable for SET valve amps, which have an easier time with closed box.

But in general, Butterworth slopes of whatever order tend to flat impedance. Linkwitz-Riley gets the familiar "omega" impedance peak, best equalised with an LCR as in this closed-box example:

I just don't think you can lose the bass peak, so an amp with significant output impedance has less control of the bass.