It's a good question. The best answer I can provide is you ought to experiment. I'll tell you about my experience. Yours could end up being different.
With a single 800, I had the 800 knob set to 80Hz which was the same as the Onk's cross-over. Wide open absolutely sounded not that good.
With the 800 and dual 500s, I ended up with the 800 at 40Hz and the dual 500s at 80Hz. Again, wide open did not sound that good.
With the 800, dual 500s and dual 600s, they are all set wide open (150Hz) and the Onk is set to 80Hz. It sounds more full, more transparent and more "musical" that way. It's the best I've ever heard anywhere.
So let your listening be your guide. Ultimately of course, equipment and listening is ideal.
