The simple fact is no ’full range’ driver is really full range. Most struggle to get below 50Hz in an optimal enclosure. So if you want full range you will need a subwoofer.
They also are beamy on top. So only one person can sit in the sweet spot and if you want to hear everything you have to keep your head pretty still. For that reason a tweeter is nice, perhaps rear-firing, to at least correct tonal balance if you are off-axis. Nicer if forward firing with the main driver so you get better dispersion.
If there is bass excursion on the driver you get something called ’Doppler Effect distortion’ because a bass note causes the cone to move relatively slowly while much higher tones are also being produced. Those higher tones will be varying in frequency up and down as the bass notes move the cone back and forth.
This causes the presentation to sound congested.
To solve this you don’t play the speaker very loud or don’t play complex material with bass.
If you can get the bass off of the cone it will sound better immediately. So you can see that you need a crossover to keep bass out of it and something for the tweeter as well.
Put simply its best to think of them as ’extended range’ drivers at best. If the speaker is lacking a crossover you know its compromised. I know a lot of single-ended advocates will not like what I’m saying here. But I’ve had a lot of experience with these drivers over the last 20 years. The lack of a crossover is worse for the speaker than a properly designed crossover.
If you use subs, they should not be active over about 80Hz else they will attract attention to themselves so you really only need to go down to 60Hz or so and you can get a good blend. But you have to sort out how to prevent excursion due to bass notes! The lower you allow the driver to go the harder this gets.
IMO if you want full range, you start with a 15" driver of some kind that allows you the bottom octaves and cross over at 400-500Hz to your extended range driver and then roll in the tweeter at about 7-8KHz.