This is definitely an interesting one. Imo, if all you can do is a Frankenstein, then that’s all you can do. I’ve had some like that. While not perfect, they were definitely listenable and fun. I definitely agree with the guys about matching. Ime, it absolutely does make a difference.
But don’t let great be the enemy of good.
For matching a center? Ryan’s point is solid. Those older JBL’s had a sound for sure. So as pogre said, contacting JBL, (or iirc there’s a vintage Harmon group. Can’t remember the name) for a recommendation would be good. Best option would be to find a third one(or iirc they just released an updated version which might match). Then let mcacc do it’s thing and see how you like it. I’ve never liked Phantom center mode, but could work if you stay on axis(centered)
I think what you might try is use the current pioneer, and MK books as you mentioned. Try phantom center, or run to the pawn shop, or check Craigslist for an L100, something else to try and just see what you can do.