I was well aware that the 416 was designed to work in ported cabinets, though as I recall from an Altec datasheet that bass down to ~ 35Hz was said to be attainable in a 7 cu ft sealed cabinet. But despite my limited knowledge and DIY skills, when these were built in 2016 I was following the lead of fellow diyaudio.com member Gary Dahl, who in pursuit of reducing distortion to very low levels (as shown in Crowe's test report) put the 416s in those sealed 3 cu ft boxes to limit their low end response, but then extend system response using Acoustic Elegance subs/passive radiators. His system as of 2018.
https://galibierdesign.com/wp-content/uploads/wa-trip_gary-d.jpg
Gary shared his cabinet sketches and numerous photos of cabinet bracing and other details so Jim Salk could clone them. Jim also built my Rythmik subs.
www.salksound.com
I recently heard that Gary only uses the subs for home theater. For music, he claims to get ample bass response from room gain due to fortuitous speaker placement. I doubt if I will get that lucky in my room, hence my questions about using the Rythmik subs.
My main concern is if there will be any big tonality differences between the 416s and these subs if I'd have to cross at around 80Hz. I suppose the only way I could cross lower would be if I was satisfied with SPLs from the Altecs at ~ 70Hz in my 2660 cu ft room, however likely that would be.
en.wikipedia.org