It's possible that you may need to reapproach this question. You see, the playing field here is uneven. I don't know about Brian McKnight, but I can guarantee you that Justin Bieber and Usher have at least one leg up on you: The leg of technology and studio production.
Ever listened to some random singer in person and heard them switch into falsetto? Often times, if that singer is a man, you may find yourself thinking, "Wait a minute, that voice just got softer, but also a lot quieter". Especially for male singers singing in a fairly high range in their chest voice switching into the low-to-middle falsetto register, you might notice a significant volume drop.
Falsetto is quieter than chest voice, most of the time. Sure, the rare singer emerges who can deceive even fellow singers as to the register in which they're singing, but for the average guy, it's not as subtle of a shift.
So, the question is, how do the pros sound so smooth and connected and flawless? Are Usher and Bieber some kind of falsetto deities? And the answer to that, my friend, is no.
I'm not saying that they're not good singers; they've got to have at least some singing ability to have become so famous in the first place. I'm just saying that the whole recording industry is doing their best to make these guys sound good. For example, blending one's falsetto and chest voice is something that even professional singers struggle with (well, usually the men, anyway). Listen to male Broadway singers: there's usually a marked difference between the falsetto and chest voice, and those guys sing for a living live on stage with little to no studio trickery and usually no released album to point to as proof of their vocal skill. It's not as though the volume drop is because they're bad singers.
My somewhat educated guess is that one thing that studios do to recordings is to boost the volume of any notes in falsetto. That goes a long way in disguising the switch, since the only difference would be the timbre. Studios have access to the ability to change timbre in post (postproduction), but I'm not sure whether that's actual industry practice. I kind of doubt that. Plus, Autotune. I don't mean to jump on the "pitch-correction sucks" bandwagon, but falsetto is also a lot harder to control than the chest voice, and when you don't need to be as specialised or have as highly-developed of a falsetto to use it in a song, it's a lot easier.
(Here's the answer to your question as presented)
That doesn't mean that you need to use technology to "cheat" your way to a smooth falsetto. My best advice for that would be practice changing the timbre of your voice. You said that you can do the rock-style falsetto, right? Well, it's not really any harder to get the falsetto to sound less piercing. When you yawn, do you use falsetto? Some people do, and if you can sort of make that sound, that's pretty much the way you want to be making that more "classical", smoother falsetto. Once you get it, you should be able to change between the two falsettos very easily, as they're on opposite ends of a spectrum. The "whinier" one feels "closed", and the lighter one feels more "open" as you make it.
Volume will come with practice (So much of singing is practicing, it's unbelievable), but things that help are not putting big belted chest notes right before falsetto if it can be helped (few songs do that anyway, luckily), and singing higher notes than normal when in falsetto. The higher the falsetto note is, the louder it can be (usually).
(opinion part)
On the topic of Usher and Bieber: While Usher probably is indeed a very good singer with a pretty well-developed falsetto tone, I find it harder to believe the same about Justin Bieber. A former child singer in the late 2000's, Bieber was more well-known for his adoring fans than for singing talent, at least among musicians. Rarely do very popular teenage singers have the voices their albums do. Bieber, being so popular at such a young age, had a whole lot of production at his disposal in the studio, and as a performer, it likely was never a huge conern for him to develop a better voice. Not saying that he didn't try to become better, just saying that it really wouldn't have mattered that much to his audience whether he really was an amazing singer. As he matured, his more recent return to the popular music scene continued to be extremely well-produced, making me question his singing abilities behind the smoke and mirrors.
I may seem as though I'm just picking on Justin Bieber; I'm not. A lot of the same things I'm saying about production value and Justin Bieber's voice apply to just about every singer on Top 40 Radio today. Youtube videos exist of "Autotune vs. Real Voice" compilations for countless famous performers and the difference is staggering, especially in the relative lack of control displayed at live shows. If you ever get the opportunity to go see a famous singer live, and their vocals are sounding really good, try to see what sounds different from the recording, since a lot of modern singers do not really sing their songs during some live performances. This applies to just about everyone in the industry; it's simply how the industry works. That's not to say that they've all got not talent, it's just that they are not chosen specifically for having the best singing voices on the planet; it is the job of the singer and the postproduction team to create that illusion.
In a similar story, current teen singer/social media personality Jacob Sartorius is widely believed to be much, much worse than his released songs would let you believe. He really does not seem to be able to perform live anywhere near his recorded standards. In particular, his performance at the recent Teen Choice Awards (saw it on TV) did not impress me, to say the least.
Another obvious time that this is apparent is when famous actors "sing" in modern movies. One example that comes to mind is Disney's blockbuster film "Moana". Dwayne "The Rock" Johnson could not sing "You're Welcome" nearly as well as what audiences heard, I guarantee you that.
Ultimately, my point is that when singers try to compare their voices with those of contemporary pop musicians, they must take into account the fact that they are at an inherent disadvantage. Pop singers tend to be very good at certain things within the scope of singing, but no one expects them to sound as good as their songs do, and falsetto is no exception.
Keep in mind that pop singers are not reasonable standards to compare yourself to.
