chorus,
You obviously have more experience and knowledge than I did when starting audio DIY stuff, so I’m sure what you do will be a success.
Given how many acoustic panels you built, I image you have some on the ceiling. Since I had horns for the upper frequencies, I figured their vertical directional control would be sufficient. But bass traps in four corners and first and second reflection point treatments making the type of difference you experienced, added some for the ceiling’s reflection point. It was so helpful I added two more behind them - not as much of a difference, but still worth it.
I don’t have experience with sealed cabinets, but did appreciate the ability to tune the ports on mine - I used two 4” round ports with adjustable length tubes vs. the cutout in the baffle Altec used on the Model 19.
“perkri" offered some good advice and port tuning is another way you can tailor things to your taste you can’t do, (or nearly as easily), with store bought.
With the Altec Model 19, some said raised the tuning frequency of the Altec 416 woofer enhanced the midrange with a just a little sacrifice of low frequency reach. Sounded good but preferred the balanced sound with cabinet tuned to Great Plains Audio’s suggested 29 Hz - just 4 Hz difference.
You’re right on the mark about multi-directional bracing in the cabinet, especially front to back in the center.
You may already know which capacitors you'd use, but in case you didn’t see this article: https://www.humblehomemadehifi.com/Cap.html.
After trying 3 different caps, the descriptions he gave were quite consistent with what I heard and a noticeable improvement over stock grade.
There’s snow on the ground here, but my JBL 29AV speakers do a decent job taking care of outdoor music. Still, you’re right, there’s nothing like a large, high efficiency speaker with proper power played outside, especially with a well recorded live album. So enjoy your music indoors and out.