Why so few speakers with Passive Radiators?

As I look back on various speakers I've had over the years, the very best sound, particularly in lower registers, came from sealed designs. This was true with big living room speaker systems; and it's true in the last 2-3 powered monitor set-ups on my desktop/nearfield system. It's also true with subs, where I find sealed to sound best.

Yes, this assessment is 100% subjective; I've done no testing apart from whatever my ears reveal to me.

Currently in my desktop setup, I've just purchased a high-quality 2-way speaker, a sealed design, said to have particularly bass for a speaker this size (ATC SCM12 Pro). That, plus a class D amplifier recently purchased (strongly influenced by positive reviews/comments regarding its musicality) will be my first amp+passives experience on the desktop. I have high hopes. In the case of the ATC, not only do I expect cleaner/clearer/tighter bass than w/previous ported & powered monitors--but hope to also get some of the renowned ATC clarity & detail in the mids & highs (said to be accurate by not bright).

I was relatively interested in several powered studio monitors that feature passive radiators, but feel I might do better with audiophile passive designs (this ATC can be considered a hybrid--it's the professional version of their very similar "hifi" model, the SCM11 v2, but reviewers usually comment on its "musicality" (an extreme rarity in the pro world to see that word).

Wish me luck!

My first pair of what I consider to be good speakers were Klipsch kg 4.2. They have a 10 inch woofer and a 10 inch passive radiator and no port. They blew away my old pioneer 16 inch mush-woffer speakers with deep, tight, and powerful bass. They are definitely heavy on the bass.

Lots of good information here about the various trade-offs of bass frequency production. I'll add one missing element of high consideration for some designers, which is system efficiency / sensitivity. The hard limit of bass output in a system establishes the overall system efficiency. If you want deeper bass, you must lower the efficiency, etc. etc. If a designer (like Jim Thiel) wants overall higher system efficiency with maximum bass extension, then he chooses a non sealed box configuration of some sort. The passive radiator takes less internal cabinet volume, doesn't chuff, minimizes internal cabinet noise, and generally behaves more predictably than an open vent.

In the particular case of Thiel Coherent Sources, the active woofer must time-align with the midrange along the baffle . . . whereas deep bass benefits from near-the-floor coupling. The radiation plane of the passive radiator can be both time-aligned with the woofer and floor-coupled. By the way, years of work went into optimizing all the elements of the vented enclosure, eventually replacing the port of the O2 and O4 with a double-surround precision passive radiator of our own design and manufacture in the CS 3.6. 

By the way, Jim Thiel (and all of us at Thiel) preferred the active equalizer solution present in the O1, O3, O3a - b, CS3 and CS3.5, which provides sealed-box precision and rolloff characteristics while extracting the needed power curve from the driving amplifier, not to exceed power required for midrange transient peaks. The active equalizer was abandoned not on technical grounds, but on persistent resistance from audiophiles. Thiel's execution was qualitatively different than Bose's, but the audiophile community didn't see that. Also, the cost to do it well was judged by the market as too high. If I were designing (which I am not) a Jim Thiel Tribute Model, it would include sealed box bass with convoluted chamber (some form of transmission line) and an active electronic equalizer. 

Your room is the most critical element of bass enjoyment. Enjoy.