For low bass extension try the Fisher XP10. You will need 80 plus watts minimum and a tweeter swap of you taste. The crossover is fine.
If you're down on power then try the Wharfedale W70D. Also with a tweeter swap. The crossover is fine.
I would argue that my Rectilinear IIIs seem to have more low bass extension than my W70Ds. They do need more power to really shine though. I have the W70Ds hooked up to a Pilot 654 and concur that they work well with "just" 7591s. W90D should be even better. I did change the tweeters for Morels as the original purple domes fell apart and cracked before I got them.
I would say the crossovers are fine... IF RECAPPED... pretty sure both the Wharfedales and Rectilinears used those miserable black with red end caps capacitors so they're about 10 years at least overdue for replacement and I at least could hear the diff. (in the Rects at least, I never listened to the Wharfedales as I got them as a project.)