Sounds like you have been on talkbass. These "fearful" cabinets are actually an amateurishly simple reflex cab design with a passively crossed over tweeter, probably better suited for a small venue high pack in a weekend warrior SR system. Not dissimilar to the homebuilt Hi-Fi speaker designs of the late 1950's - early 1960's. Designed (probably on WinISD freeware) several years ago by a "wanna be" faux audio engineer who offered the plans for free online.
Through self promotion this fella gained some traction as an "internet expert" about transducer science on talkbass, mostly among the neophyte bass player members that really had no other frame of reference in the real world of audio engineering, and a cult like devotion to his "fearful" designs grew over a few years. As a fan of low/no cost marketing myself, this in and of itself is just dandy as far as I'm concerned.
It irks me (obviously), not because someone bulk posted on a free forum promoting his beginner level, freeware design as the greatest bass cab ever conceived; as I said that's just good marketing in my book, but because this wanna be and his follow the leader cult also vehemently bashed any thread about bass cabs that were NOT his bush league design. That type of behavior is classless and rude even from folks who actually know what they are talking about, and extremely disruptive and counter productive from posters who don't have a clue what they are posting about, and just parrot what they have seen other post on the subject in an attempt to readers that they do know about the subject.
As I said earlier, in the MI world it's about sounding good rather than what the actual response characteristics of a cab are; as opposed to in the Sound Reinforcement world where technology constantly improves the state of the art. Classic MI cabs were largely designed "by ear", because obviously the technology that pretty much everyone now has at their fingertips wasn't available then. For that matter, many iconic, most popular and still used MI cabs were designed before Mr. Small even began expanding on Mr. Thiele's research and adapting it to early computer models.
Many of the modern cabs that are highly regarded and commonly used in the professional world of live and studio Musical Instrument amplification still rely on ears for the final testing, although modern design software saves a BUNCH of trees compared to the old "build it and try it" R&D methods.
All that said, it's in the ear of the end user where the decision should be made, there is no "best". "Best" in what sounds good is like saying what the "best" color is or what the "best" flavor is, can't be done. As I said, what irks me about the "fearful" thing isn't the cabinet itself, I'm not a fan of passive crossovers or tweeters in a bass guitar cabinet from a technical standpoint, but I don't doubt that many players are satisfied with the performance of the cabs. It's the attitude among its supporters that fearfuls are the end all/be all BEST bass cab out there, and using or preferring anything else makes a person ignorant and/or unenlightened and needing to be "corrected" in an open forum.
In fact, if a bass player feels the need to go the full range accurate route with a bass rig, the most efficient route would be to invest in a commercially produced powered full range SR cab. Even the inexpensive weekend warrior class equipment absolutely destroys any fearful design from a sound quality, power to weight, or cost efficient angle. Latest and greatest design technology, uber light, nearly indestructible polymer materials, on-board megawatt and accurate class D amps, neodymium drivers; the "big league" version on what the "fearful" design is the homemade "beginner designer" version of, and can be bought for dirt cheap.
Back the the Sunn cabs, they are very flexible as far as drivers. The only exception to that would be the 200S's restrictive port ducting that can make some larger ceramic magnet or deep frame drivers difficult to fit in a traditional rear loading configuration. Tonally I probably like the tone and response of the AlNiCo JBL's in Sunn cabs, but from a practical angle several more current drivers fit the bill quite well.