I've been fooling with Winisd to build a 2226J and 2123J woofer and mid with the Vifa 25 on top.
Winisd and your 4 Pi Pro are close in size, about 3.6 cu ft for the 2226 tuned to 40Hz.
But in the referenced post you have the 2226 in a 2.5 cu ft box.First, which box size is better? 2.5 cu ft shows a head and shoulder between 60 and 90 Hz. 3.9 cu ft is smoooth.
Second, where would you cross to the Vifa?
Third, what about the 6dB lower efficiency of the Vifa? Does it matter?