I have looked at the original design to remind me about its qualities.
The component list looks good and as you say the SB speakers are more than good for the money. So It seems a considerable amount of time and effort has gone into the design. When you publish a design to sell, I imagine its a very daunting thing as there will always be people who want to pick holes in everything. The big but, is speaker design is about many things, the design aim, cost, your interpretation of the aim, and then listening and tuning to match. Not forgetting rooms will modify every design. As will the number of people in your listening room, record shop or wine bar where you use your speakers
Your Xsim data suggest to me that basically the design should sound good on axis with both speakers basically pointing at the listener. That's according to my criteria and what I currently understand from the graphs.
If you have the time and inclination and you want wider cabinets, it would be interesting to come away from current fashion and go with a 70's style wider baffle. I grew up with wide baffles from the likes of Wharfedale, Celestion, Goodmans, Kef, B and W, Acoustic research, the odd Snell, and B and O designs. So I am pre disposed to them.
Wharfedale Dovedale's , Leak 2060's a more modern Dovedale, Celestion Ditton 44 and Yamaha NS1000s will always bring back happy memories for me. Interestingly they are all sealed three ways if I recall correctly, 12" bass, wide baffle to accommodate the 12" driver 60 litres or so. Midrange and tweeter. One day, maybe next year, I may do a design myself but I haven't landed on the right 12" bass yet. And I do want to try that new Sb dome. Hint!
Consequently the other advice about a three way could be another path to investigate if you want to design something new.
Where you live is there a classic 12" bass unit that was used by everyone in the 70 and 80s in a sealed box. If so buy some good second hand examples and then integrate with SB's latest 3 or 4" midrange in a separate small sub compartment within the box. If you are really lucky Sb will have released their new 50mm dome, then add in your current tweeter, its a good one. With sensible design from yourself it will sing. But can you live with that size and shape of Box.
Good luck.
