Wow, nice hardware. Those good 515s can sold for lots of money.

I would do an active crossover. 20 years ago, I built a set of speakers using 2 Altec 414s, 806/811, & EV T350s per side; and I used an active crossover from DeCousey(btw they are long gone). I used 800hz & 7khz with 12 db slopes. In hindsight I should've used 18db slopes. I just used a 60watt/ch SWTPC(Southwest Technical Products Corp) and 25watt/ch SWTPC that I soldered together from a kit, and a Crown DC150 for the woofers.

They sounded so good to my ears. Back then all of the instrumentation was in the realm of labs and universities.