While we agree that many lower cost speakers lack uniform and deep bass and that subs certainly help in those cases, remember that large high efficient speakers mated with the proper equipment can excel in deep, fast, and accurate bass reproduction. That’s my point since I have speakers like that, I no longer feel the need for subs as my speakers, and others with similar technologies can play deep bass well enough without worrying about that last iota of bass...been there done that. It is not as simple as saying thou shall must have subs because...it’s a matter of the efficiency of the equation of the room, electronics, and speakers. That’s where I am at, an sufficient level of performance that negates the use of subs for my applications. But, as many have stated, subs are most likely needed if you can’t get the ingredients right.
The problem with nearly all high-efficiency bass apparatuses is they lack extension. There may be some exceptions out there but I’m not aware of any non-custom speakers short of six figures that really fall into that category. Maybe some Tektons?? Wilsons are a good example of an otherwise efficient speaker (relatively good sensitivity) if not for the tough load of their bass woofers. Unfortunately, the excursion required for the lowest notes results in an inefficient driver, which is why many speaker designers choose to use active/powered bass systems. Speakers that extend to ~35Hz are often referred to as “full-range,” but they’re not really if we’re honest. I’m not saying they can’t be enjoyed without subs, only that they can still be enhanced with a well implemented sub. IME, the lowest two bass octaves do enhance the rest of the bandwidth for some reason, and can greatly improve sound-staging cues.
Your custom 400lb. behemoths are outliers, obviously.
There’s nothing wrong with living without subs if that’s one’s thing. I guess my point is that if someone is seeking full-range sound on a budget, it’s often better to get high-quality standmounts with stereo subs, as you end up with better quality bass sections than those found in some $10K/pair^ towers, and you also get true full-range extension (assuming a quality sub).