Saki70, you have very good questions. I'm going to avoid getting too specific here because I don't want my post to cross the line and become an "ad".
Driver integration is dependent on driver vertical spacing, crossover frequency & slope, and listening distance. Briefly, the ear is poor at resolving the height of a sound source below about 1 kHz, improves dramatically between 1 kHz and 4 kHz (where it peaks), and then actually decreases a bit at higher frequencies but remains pretty good. Steep-slope crossovers give better driver integration (less vertical smearing) at close range than shallow-slope crossovers. In my experience, having a suitably low crossover frequency is more beneficial to driver integration than is close inter-driver spacing. Perhaps Clio09 will post here, as he's the one who really opened my eyes (and ears) on this subject.
Saki70, you mentioned that in a larger room we want to crank it a bit but in a smaller room we want to turn the volume down. One factor that comes into play here is the thermal modulation characteristics of the drivers. Usually the tweeter is more efficient than the woofer so it's padded down, and it normally gets a lot less power anyway. So many speakers run into the problem of the woofer's voice coil heating up more than the tweeter's voice coil, so the woofer has more thermal compression as we turn the volume knob higher. This voice coil heating is virtually instantaneous; a 100-watt transient is like touching a 100-watt soldering iron to the voice coil. Anyway, if the woofer's voice coil is heating up faster than the tweeter's it will have more thermal compression at high input levels. The designer then has to choose an input level at which the drivers are balanced relative to one another, and he will probably choose a fairly high level, let's say 90 dB/1 meter for this example. If we go to 100 dB, the tweeter will get loud a bit faster than the woofer because its thermal compression is less, so the speaker will sound a bit bright on peaks. If we go down to 60 or 70 dB, now the tweeter is softer than the woofer so the speaker sounds a bit dull and lifeless. I think this phenomenon is behind the fact that many speakers do not really "come to life" until you crank 'em up a bit.
So to sum up the preceding paragraph, it's not uncommon for a speaker's tonal balance to change with the input power level, going from dull at low levels to "just right" (the Goldilocks zone) to too bright at very high levels. If a speaker is going to work well at a wide range of power levels, either the woofer and tweeter need to have very similar thermal compression characteristics or their departure from linearity should happen at higher input power levels than we're likely to see in the home.
On a related note, low thermal compression correlates with speakers that convey emotion well. Musicians use variations in loudness to convey emotion, and it's nice if the speakers can preserve those dynamic shadings. If a speaker is compressing the peaks by 2 or 3 dB, well then ou lose emotional impact.
So anyway in order to meet the requirement that a speaker work well at low levels in a small room and also at high levels in a large room, a lot of issues have to be addressed. I think it's easier to address them with prosound drivers, but won't claim that's the only feasible approach. But large-diameter prosound woofers and high quality constant-directivity horns & waveguides combine good directional control with the ability to move a lot of air should they be called upon to do so.
I don't think it's possible to design a speaker whose characteristics in the bass region do not change significantly as its position in the room (and/or the room itself) is changed, so I think the best we can do is build in a reasonable amount of adaptability that will hopefully cover most situations.
Duke
Driver integration is dependent on driver vertical spacing, crossover frequency & slope, and listening distance. Briefly, the ear is poor at resolving the height of a sound source below about 1 kHz, improves dramatically between 1 kHz and 4 kHz (where it peaks), and then actually decreases a bit at higher frequencies but remains pretty good. Steep-slope crossovers give better driver integration (less vertical smearing) at close range than shallow-slope crossovers. In my experience, having a suitably low crossover frequency is more beneficial to driver integration than is close inter-driver spacing. Perhaps Clio09 will post here, as he's the one who really opened my eyes (and ears) on this subject.
Saki70, you mentioned that in a larger room we want to crank it a bit but in a smaller room we want to turn the volume down. One factor that comes into play here is the thermal modulation characteristics of the drivers. Usually the tweeter is more efficient than the woofer so it's padded down, and it normally gets a lot less power anyway. So many speakers run into the problem of the woofer's voice coil heating up more than the tweeter's voice coil, so the woofer has more thermal compression as we turn the volume knob higher. This voice coil heating is virtually instantaneous; a 100-watt transient is like touching a 100-watt soldering iron to the voice coil. Anyway, if the woofer's voice coil is heating up faster than the tweeter's it will have more thermal compression at high input levels. The designer then has to choose an input level at which the drivers are balanced relative to one another, and he will probably choose a fairly high level, let's say 90 dB/1 meter for this example. If we go to 100 dB, the tweeter will get loud a bit faster than the woofer because its thermal compression is less, so the speaker will sound a bit bright on peaks. If we go down to 60 or 70 dB, now the tweeter is softer than the woofer so the speaker sounds a bit dull and lifeless. I think this phenomenon is behind the fact that many speakers do not really "come to life" until you crank 'em up a bit.
So to sum up the preceding paragraph, it's not uncommon for a speaker's tonal balance to change with the input power level, going from dull at low levels to "just right" (the Goldilocks zone) to too bright at very high levels. If a speaker is going to work well at a wide range of power levels, either the woofer and tweeter need to have very similar thermal compression characteristics or their departure from linearity should happen at higher input power levels than we're likely to see in the home.
On a related note, low thermal compression correlates with speakers that convey emotion well. Musicians use variations in loudness to convey emotion, and it's nice if the speakers can preserve those dynamic shadings. If a speaker is compressing the peaks by 2 or 3 dB, well then ou lose emotional impact.
So anyway in order to meet the requirement that a speaker work well at low levels in a small room and also at high levels in a large room, a lot of issues have to be addressed. I think it's easier to address them with prosound drivers, but won't claim that's the only feasible approach. But large-diameter prosound woofers and high quality constant-directivity horns & waveguides combine good directional control with the ability to move a lot of air should they be called upon to do so.
I don't think it's possible to design a speaker whose characteristics in the bass region do not change significantly as its position in the room (and/or the room itself) is changed, so I think the best we can do is build in a reasonable amount of adaptability that will hopefully cover most situations.
Duke