Hi,
I thought I would share an update. After purchasing the parts for the modification I added 2 JBL 2226H 15” woofers as subwoofers coming in at 80 hz 12 db slopes. This has been an amazing improvement. The room they are in is roughly 8 m x 8 m x 2.75 m.
The sound is perfectly balanced now. I love the amount of resolution I am hearing with such a pleasant presentation. I believe Ill be leaving the crossovers as you designed them and maybe revisiting the mod if they are moved to a smaller room. Hopefully I will get around to veneering them.
The JBLs are also my first experience with woofers larger that 8”. I understand now why so many people insist on large woofers. I would say I was unaware of how much information is recorded that simply requires surface area to be resolved. Specifically the harmonics and tone of instruments playing that low. Thank you for all you put out to read and think about and the great plans you make available. 
Thanks again,
Marshall