Hello Scott,
Assuming the same impedance and SPL/W curves as the D2430K, here are equivalent networks that should result in same high-pass frequency and overall sensitivity (ie noise) :
Code:
32 ohm driver ( 20 ohm load, 9dB attenuation): R1=13 (30W) ; R2=11 (10W) ; C=8
16 ohm driver ( 10 ohm load, 12dB attenuation): R1=7.5 (75W) ; R2=3.3 (20W) ; C=16
8 ohm driver ( 5 ohm load, 15dB attenuation): R1=4.1 (160W) ; R2=1.1 (30W) ; C=32
4 ohm driver (2.5 ohm load, 18dB attenuation): R1=2.2 (350W) ; R2=0.36 (40W) ; C=64
As you can see resistors rating go up like crazy...
Alternatively you can use an autoformer (like the ones found in cheap in-wall volume controls) to simply turn the 5 ohm load into a 20 ohm one and use the original passive network.
By the way, Truextent diaphragms also come in 16 ohm, and that is probably a better choice than 8 ohm in your situation.