I think you're going to find that the 4675C doesn't need a bunch of fancy DSP. An L-R 24 crossover (3rd Order on the HF, 4th order on the LF will work best with that horn/driver on the HF) with the right delay on the LF and some CD horn boost and you are pretty much going to have a pretty flat response. In the cinema industry, there are a TON of analog crossovers for that speaker because it was so popular. If you like QSC, their MXa amplifiers had the XC-1 crossover, the EX amp had the XC-2 the current DCA (though CX will work) can use the XC-3 crossovers. The Series 1 amps had octal plugs and both QSC and a company called USL (now, recently, owned by QSC made their XTA series of octal plugs as well as a frame that took those octal plugin crossovers). One of the last vestiges of those crossovers that had the JBL 4675C in mind is the XTM-300 by USL (now QSC)
https://www.qsc.com/cinema/products/.../usl-xtm-300a/
Component Engineering may even have some of their cards (fit a dbx 900 series frame, as I recall or the Genuine THX crossover/monitor (analog)). Those cards will be for the 4675C https://www.componentengineering.com/thx_crossover.html. There are a ton of the D1138 crossover/monitors on eBay at the moment, and they will work but they are EARLY dsp audio and you'd need the correct DOS program to configure it (yes, I said DOS). I would tend to stick with a good analog crossover myself unless the DSP is coming along for other uses.