I am not as experienced with the 604D as I am with the early ones and the later "G" models...

But the 604D is a fine speaker. If the Alnicos are fully charged, the only place a real improvement on the speaker could be made is the horn. I don't care for the small multicell.

Crossover is a different story. If you do a forum search here, you'll find links to better DIY crossovers, as well as many other ideas.

The stock 604 crossover needs improvement. I would think about that, rather than getting new speakers, and just make sure all is working well on the speakers, ie magnets, frams, etc