I can put in a summing circuit easy enough, but most of the circuits I have looked at combine summing with an active crossover, which increases the complexity quite a bit.

What I may try is using a passive xover with a summing circuit either before or after the xover.

On a 4 ohm load, what resistor value (if any) would be appropriate to but in series with the capacitor on the low frequency side (second order xover)? I'll be crossing over at about 100Hz, IIRC, so the cap is about 500uf.