Danny we high jacked your thread. What would you do to solve this problem?
Wide band driver, with support at top and bottom? No crossover in the critical regions with good phase coherence? Perhaps V2 bass as all agree it works and is economical.
This is going to be a bit of a complex answer.
Here is the way I look it speaker design. It really is about speed. Speed can mean two things. It can mean how fast a driver returns to rest or how fast it can move.
How fast a driver returns to rest is what people are referring to when they say the bass is fast. This can be achieved several ways. Multiple smaller drivers can be used like in a line source or an array like the NX-Treme. With many drivers handling the lower end they each move less so there is less inertia or stored energy to deal with. And smaller drivers means less moving mass. Less moving mass can also speed settling times up. And then for really low end duty there is servo control.
Speed can also refer to how fast a driver can react or track an input signal. If you want to know how fast a driver is just look at how high it will play. If it will react fast enough to play the shorter wavelengths in the top octave then that's pretty fast. If it is not fast enough it simply won't play up very high.
Now I said all of that to get to another point.
Longer wavelengths (low bass) requires more surface area and longer exertions to maintain SPL levels down there. If a single driver is having to do everything (meaning play from 20Hz to 20kHz) then it is having to handle long exertions and everything else all at the same time. Imagine a driver having to handle the top octave while at the same time the cone movement is being dominated by the first two octaves. It becomes a compromise of everything. And the longer exertions mean longer settling times. So speed is hard to maintain down low.
The solution of coarse is to limit the lower end that the driver has to cover. If you just remove the first octave then you have relieved it of about half of the demand on it. Move up one more octave to 80Hz and now you are really taking a load off. The driver can now much more effectively handle everything else.
The upper end of the sweet spot tends to be just below 200Hz. If you take that part off of a driver then you have taken over 80 to 90 percent of the exertion requirements away from it. And along with that a lot of potential stored energy is gone. Setting times are much faster and cleaner and they can more effectively play everything else.
Now take that same line of thought and move it up the scale. Tweeters are much lighter and faster. They have a lot less moving mass as well. Smaller diameter also means better dispersion and off axis response.
No matter how great a full range driver really is it will still get clobbered by a good tweeter in the upper ranges.
So let the tweeter do what it can do. And the lower the tweeter can play the more speed you bring into the lower frequency ranges. This is why I like lower tweeter crossover points. And a Neo 3 in a waveguide crossing as low as 1.3kHz is hard to touch. Now the mid-bass driver doesn't have to be as fast.
And the real range to avoid a crossover is the mid-range. The heart of it is in the 300Hz to 500Hz region. I like to stay below 200Hz and just above 1kHz ideally.
Again it is all about speed and you can see it in the spectral decay.
And yes, drivers like the Neo 3 and Neo 10 are super fast. There' s nothing like them. The Wedgie design using four 3" LGK drivers and the Neo 3 is also super fast. The NX-Otica and NX-Treme models use the M165NQ drivers as mid-bass drivers and don't give up much speed in exchange for a little more body. All of those are favorites of mine.
So as for full range drivers. Some are really good. The LGK's are great as they are small (3" driver) and fast with decent off axis response for a full range driver due to its small size. The compromise is a limited SPL.
The TB has an incredibly smooth response after the filter. But again there are compromises. Off axis response is of coarse very limited. It can play up so high because of the smaller wizzer cone. The main cone is too heavy and damps out the upper ranges. If you had them already in use and wanted to take them up a notch then you could do so by properly implementing a rear firing or upward firing tweeter as a super tweeter. But if adding a tweeter you might as well configure a real crossover and let the tweeter do what it does best. Going at it from the beginning to add a tweeter would have me going for a better mid-bass driver.